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When you receive an Error Message Rejected when trying to send an email, this means that a spam filter has blocked the message. What's behind the SMTP error? According to the “Request for Comments ” mentioned above, the standard form of the SMTP status code. , "", "This message does not have authentication information or fails to pass authentication checks (SPF or DKIM). To best protect our users from spam.

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Fix email delivery issues for error code in Exchange Online

  • Article
  • 4 minutes to read

It's frustrating when you get an error after sending an email message. This topic describes what you can do if you see error code or in a non-delivery report (also known as an NDR, bounce message, delivery status notification, or DSN).

Use the information in the NDR to help you decide how to fix the problem.

Why did I get this bounce message?

The destination email server that generated the error won't accept messages from you (the sender) or messages for the recipient. This can happen if messages from you (your email address, your Exchange Online organization, or even all of Exchange Online) are being blocked by the recipient.

I got this bounce message. How do I fix it?

This section contains steps that you can try to fix the problem yourself.

If these steps don't fix the problem for you, contact your email admin and refer them to this topic so they can try to resolve the issue for you.

You're in the recipient's block list

Your email address could be in the recipient's personally-maintained block list. This is the likely cause if you can successfully send messages to other recipients in the same domain (for example,

Contact the recipient (by phone, in person, etc.) to verify that your email address isn't in their block list.

Remove bad entries from your Auto-Complete List

You might have an invalid entry in your Auto-Complete list (also known as the nickname cache) for the recipient. For example, the recipient might have been moved from an on-premises Exchange organization to Exchange Online, or vice-versa. Although the recipient's email address is the same, other internal identifiers for the recipient might have changed, thus breaking your cached entry for the recipient.

Fix your Auto-Complete list entries in Outlook

To remove invalid recipients or all recipients from your Auto-Complete list in Outlook later, see Manage suggested recipients in the To, Cc, and Bcc boxes with Auto-Complete.

To resend the message in Outlook, see Resend an email message.

Fix your Auto-Complete list entries in Outlook on the web

To remove recipients from your Auto-Complete list in Outlook on the web (formerly known as Outlook Web App), do one of the following procedures:

Remove a single recipient from your Outlook on the web Auto-Complete list
  1. In Outlook on the web, click New mail.

  2. Start typing the recipient's name or email address in the To field until the recipient appears in the drop-down list.

  3. Use the Down Arrow and Up Arrow keys to select the recipient, and then press the Delete key.

Remove all recipients from your Outlook on the web Auto-Complete list

You can only clear your Auto-Complete list in the light version of Outlook on the web. To open your mailbox in the light version of Outlook on the web, do either of the following steps:

  • Open the mailbox in an older web browser that only supports the light version of Outlook on the web (for example, Internet Explorer 9).

  • Configure your Outlook on the web settings to only use the light version of Outlook on the web (the change takes effect the next time you open the mailbox):

    1. In Outlook on the web, click SettingsSettings icon..

    2. In the Search all settings box, type light and select Outlook on the web version in the results.

    3. In the page that opens, select Use the light version of Outlook on the web, and then click Save.

    4. Log off, close your web browser, and open the mailbox again in Outlook on the web.

After you open your mailbox in the light version of Outlook on the web, do the following steps to clear all entries from your Auto-Complete list:

  1. Choose Options and verify that Messaging is selected.

  2. In the E-Mail Name Resolution section, click Clear Most Recent Recipients list, and then click OK in the confirmation dialog box.

  3. While you're still in Options, to return your mailbox to the full version of Outlook on the web, go to Outlook version, clear the check box for Use the light version, and then click Save.

  4. Log off and close your web browser. The next time you open your mailbox in a supported web browser, you'll use the full version of Outlook on the web.

I'm an email admin. How do I fix this?

The Sender Policy Framework (SPF) record for your Exchange Online domain might be incomplete, and might not include all sources of mail for your domain. For more information, see Set up SPF to help prevent spoofing.

Details for error code

The NDR from Exchange Online for this specific error might contain some or all of the following information:

  • User information section: Address Rejected. A problem occurred during the delivery of this message to this email address.

  • Diagnostic information for administrators section: Recipient address rejected: SPF Permanent Error.

Still need help?

Get help from the community forums.

Admins: Sign in and create a service request.

Admins: Call Support.

See also

Email non-delivery reports in Exchange Online

SMTP error messages

Connection timeout

If there is a timeout while hMailServer is waiting for a command from the SMTP client, this error message is sent to the client before hMailServer disconnects the client. The SMTP timeout in hMailServer is 10 minutes.

Excessive amounts of data sent to server.

This error is generated by hMailServer if a client sends a large chunk of data to hMailServer not containing a newline character (command terminator). A client should never do this, but incorrectly configured clients could cause this problem.

Please try again later.

This error message is issued if grey listing is enabled, and the sender, recipient and IP address triplet does not match an existing greylisting triplet.

Line too long

If a SMTP client sends a SMTP command which hMailServer considers beeing too long, hMailServer issues this error. This error typically indicates a client defect or a hacking attempt.

EHLO Invalid domain address.

This error message is issued if the domain address given in the EHLO command does not have the correct syntax.

HELO Invalid domain address.

This error message is issued if the domain address given in the HELO command does not have the correct syntax.

TURN disallowed

If a SMTP client tries to use the TURN command hMailServer responds with this error code. hMailServer does not include support for the TURN verb.

Unimplemented command

If a SMTP client tries to use a command which hMailServer has no implementation for, this error message is sent to the client.

Use HELO/EHLO first

After a SMTP client has connected to a SMTP server, the first thing it should do is to identify itself using the HELO or EHLO command. If the client does not do this, hMailServer responds with this error message.

VRFY disallowed

If a SMTP client tries to use the command VRFY, hMailServer responds with this error code. hMailServer does not include support for the VRFY verb.

Issue a reset if you want to start over

When an email client delivers an email message to an email server, it starts of by telling the server the senders address. After it has done this, a transaction is started which is not finished before the entire message has been delivered to the email server. If the client wishes to abort the transaction and send another message, it should issue the RSET command. If a client, in the middle of a transaction, tries to send a new email without first aborting the current transaction, hMailServer issues this error message. This indicates a bug in the SMTP client.

Must have sender and recipient first.

When a SMTP client is delivering an email to a SMTP server, it must specify both the sender and recipient before trying to submit the actual message content. If a client fails to tell hMailServer the senders or recipients address prior to trying to submit the message content, hMailServer will respond with this error. This indicates a bug in the SMTP client.

Must have sender first.

When a SMTP client is delivering an email to a SMTP server, it must specify both the sender and recipient before trying to submit the actual message content. The client must first tell the server the sender address and after that the recipient address. If the client tries to tell hMailServer the recipient address before the sender address, this error message is issued. This indicates a bug in the SMTP client.

Authentication mechanism not supported.

This error message is issued if a client tries to authenticate using a method which is not supported by hMailServer.

Authentication type not specified.

This error message is issued by hMailServer if a client tries to authenticate without specifying authentication method. This error indicates a bug in the used SMTP client.

SMTP authentication is required.

You have enabled SMTP authentication for the IP range that the user is connecting from, but the user has not configured his client to use SMTP authentication. There's two ways to solve this problem. Either configure your email client to use SMTP authentication. This setting is normally found in the account settings in your email client. Or, disable SMTP authentication for the IP range. The first solution is recommended since it reduces the risk that anyone will send spam through your server.

By default, hMailServer does not require SMTP authentication for connections coming from localhost / For connections coming from other hosts, SMTP authentication is required for deliveries to external recipients. By default, hMailServer never requires SMTP authentication for deliveries to local accounts, since that would prevent other e-mail servers to deliver email to your installation. For information on how to enable SMTP authentication, check the HOWTO.

If you are using a Cisco router, you may need to disable SMTP Fixup protocol. If this is enabled, the router will sometimes intercept SMTP traffic and replace data in it before it reaches hMailServer which will cause problems.

Authentication failed. Restarting authentication process.

If a SMTP client authenticates but the username or password is incorrect, or the account is disabled, hMailServer sends this error message to the client.

A valid address is required

hMailServer issues this error message if a SMTP client tries to specify a recipient address which is not valid (which has an incorrect syntax).

Account is not active.

If a SMTP client tries to send an email message to an account which is not enabled, this error message is given to the client.

Alias is not active.

If a SMTP client tries to send an email message to an alias which is not enabled, this error message is given to the client.

Blocked by SPF

If an email message is rejected during SPF checks, this error message is issued.

Delivery is not allowed to this address

This error means that the sender is trying to send an email to an address which he is not allowed to send to. This message is generated after hMailServer has checked the IP range settings. As an example, the default IP range configuration does not allow external users to send messages to other external users. This is to prevent people from using your server to send spam. So if an external user tries to send a message to another external user, he will get this message.

Distribution list is not active.

If a SMTP client tries to send an email message to a distribution list which is not enabled, this error message is given to the client.

Domain has been disabled.

If a SMTP client tries to send an email message to a domain which has been disabled, this error message is given to the client.

Invalid syntax. Syntax should be MAIL FROM:<userdomain>[crlf]

If a client issues a MAIL FROM command with an incorrect syntax, hMailServer issues this error message.

Login credentials no longer valid. Please re-authenticate.

During a SMTP session, a SMTP sender can send multiple email messages. Each time a SMTP client tries to deliver a new message to hMailServer, the client is re-authenticated to ensure that the username and password is still valid. This is needed since there is no limitation on how long a SMTP client may stay connected to hMailServer as long as it is sending messages. If hMailServer did not re-authenticate connected users, there would be no way to disconnect a user which were sending spam (without stopping the.

Mail server configuration error. Too many recursive forwards.

When an email client tells hMailServer who the email message is for, hMailServer tries to determine the "end recipient". The email address the client has given hMailServer may not be the end recipient. For example, if you have set up an alias, [email protected] which points at [email protected], and the email client tells hMailServer that the message is for [email protected], the end recipient is actually [email protected]

It is possible to configure hMailServer in an incorrect way in this area. For example, say you have an alias named [email protected] pointing at [email protected], and the alias [email protected] is pointing at [email protected] When hMailServer is trying to determine the end recipient for an email to [email protected], it will give up since there is none and report the above error message. The error will always be reported if hMailServer can not determine the end recipient.

The following causes are the most common ones:

  • A catch-all address has been specified for the recipient domain, but no account exists which matches the specified catch-all address.
  • The message is being sent to an alias which does not point at a valid account

Not authorized.

If a client tries to send an email message to a distribution list which it has not permission to send to, this error message is issued.

Recipient not in route list.

If a client tries to send an email message where the recipient domain matches a route, but the recipient address is not in the list of valid addresses, this error message is given to the client.

Sender address must be specified.

If hMailServer is configured to reject empty sender addresses, and a SMTP client tries to use an empty sender address, this error is issued.

Sender domain does not have any MX records.

If an email message is rejected due to the MX check, this error message is issued.

The address is not valid.

hMailServer issues this error message if a SMTP client specifies a sender address which is not valid (which has an incorrect syntax).

The host name specified in HELO does not match IP address.

This error message is a part of the spam protection mechanism in hMailServer. When a sending email server delivers an email message to hMailServer, one of the first things it needs to do is to identify itself. It does this by sending the command HELO <HOSTNAME> where <HOSTNAME> is replaced with its host name. The host name the sending server gives in the HELO command should resolve to the IP address of the same server.

For example, if one of Hotmails servers tries to deliver an email to your server, it will send a command similar to HELO If the option Check host in the HELO command has been enabled in the spam protection settings in hMailServer, hMailServer will check that the host name Hotmails server sent,, matches the IP address the connection is being made from. If the IP address does not match the host name, hMailServer considers the email message to be spam. If you have configured hMailServer to delete e-mail which is considered spam, hMailServer will report the above error message to the sender.

If someone tries to send you an email and you they get this error, take one of the following actions:

  • Notify the administrator of the server sending the email that they have not specified the correct host name in the HELO command.
  • Disable the "Check host in the HELO command" option in the spam protection settings using hMailServer Administrator or PHPWebAdmin. This option is disabled by default.
  • In the spam protection settings, select that hMailServer should deliver spam messages, but modify the message headers. Also select to modify the message subject. Then the email will be delivered, but the subject will be prepended with [SPAM].

Unknown user

This error message is issued if the SMTP client tries to deliver an email to a domain hosted by hMailServer but the recipient account cannot be found and no catch-all address has been specified for the recipient domain.

<Other error message>

In DNS blacklist and SURBL configuration, it is possible to specify custom error messages to be used when an email message is being rejected.

Message size exceeds fixed maximum message size. Size: x KB, Max size: y KB

In hMailServer it's possible to specify a maximum message size in the SMTP settings and in the domain settings. If a message is sent which has a size which exceeds these limits, this error message is issued.


If a hMailServer script running on the OnAcceptMessage event rejects a message without specifying an error message, this error is issued.

Your message was received but it could not be saved. Please retry later.

If the email message was received by hMailServer but could not be saved in the database, this error message is sent to the client. To resolve this issue, the hMailServer logs leading up to the error needs to be analyzed. Normally an error message with more details will be logged when this error is reported to the client. 

Rejected - Message containing bare LF's.

According to the SMTP specification, every line in an email message should be separated by the ASCII-codes 13 and 10 - carriage return (CR) and line feed (LF). Some spammers and incorrectly working software sends messages which are not correctly formatted. Use this setting to reject these messages. Please note that legitimate email might have incorrectly formatted line endings, if the sending software contains bugs.

If you who are a developer receive this problem, confirm that each line of your email message (both header and body) is ended with a carriage return and a line feed, and not just a line feed. How to do this depends on what programming language you are working with. In .C++, C#, and PHP add \r\n to the end of every line. In Visual Basic, add vbNewLine or vbCRLF.

On Windows, the default line separator is CRLF. On Linux and UNIX, the default separator is only LF. However, when sending an email message from a Linux/UNIX system, CRLF must always be used. Some email servers under Linux (such as Postfix) automatically replaces LF with CRLF. Hence, setting the line separator in the email to CRLF will cause it to be changed to CRCRLF.

Rejected - No data saved.

If hMailServer received an email message from a SMTP client but could not save the message file on disk, this error message is issued. The problem will occur if the data directory specified in is not writeable by the hMailServer service.

Rejected - <other error message>

When a hMailServer script rejects a message in the OnAcceptMessage event, it can specify an error message to be sent to the client. If a script does this, hMailServer sends this error message to the client. <Other error message> is replaced with the error message given by the script.

Tagged as Spam by SpamAssassin

If an email message is rejected by SpamAssassin, this error message is issued.

Error messages in bounce messages

The mail server software tried to deliver e-mail to the local machine

This error message typically indicates a server configuration error. hMailServer does a number of checks before message delivery to prevent infinite message looping. When an email is sent and the recipient can not be found in the local installation, hMailServer will normally try to connect to the recipients email server to deliver the email message.

Before hMailServer connects to the recipients email server, hMailServer checks that the IP it is going to connect to is not a local IP address. If the IP is a local IP address, this would mean that hMailServer would connect to itself, which would likely cause a message loop. In this case, hMailServer rejects the message delivery and returns an error message to the sender instead.

The following causes are common for this problem:

  • A host name or IP address which points at the local computer has been entered as SMTP relayer. Go to the Delivery of e-mail section in the SMTP settings. Check the SMTP Relayer setting. If you have entered localhost, ,, or something similar in this textbox, this is likely the cause of the problem. If this is the case, read more about this setting in the SMTP reference guide, and then correct it.
  • One of the MX records for the domain points at your server, but the domain has not been added to your installation.

No mail servers exists for the address.

When hMailServer delivers an email to an external recipient, it does a DNS query to determine where the email message should be delivered. If this DNS query fails, the above error message is reported. For example, the query may fail if the DNS server is unavailable or if the recipient domain does not exist. For further troubleshooting, check the hMailServer error log. The hMailServer error log will contain error codes from the DNS-client in Windows.

Error messages not generated by hMailServer

Mailbox unavailable

hMailServer never generates this error message. If hMailServer is trying to deliver an email message to another server, but the recipient account cannot be found on that server, the recipients server may issue this error message.

The error message indicates that you are sending the email to an incorrect address. If you are sure that the address is correct, it may be a problem in the recipients DNS configuration.

Sender verification failed

When hMailServer deliver an email to another server, the receiving SMTP server may try to validate that the email sender really exist. If this verification fails, it may respond with the error message 'Sender verification failed'. In these cases, the email will be bounced back to the sender. This verification works the following way:

  1. hMailServer connects to the recipients SMTP server
  2. hMailServer tells the recipients SMTP server that the email is from [email protected]
  3. The recipients SMTP server looks up a MX record for the domain
  4. The recipients SMTP server connects to the host specified in the MX record - which is likely where your hMailServer server is running if the MX records are set up properly.
  5. After this, the recipients SMTP server issues the commands HELO, MAIL FROM<> and RCPT TO:
  6. If hMailServer confirms that the recipient [email protected] exist, the recipients SMTP server will allow the delivery initiated in step 2 above.

There are a few things which can go wrong in these steps:

  • If the MX records for the domain is not set up properly, the recipients SMTP server may correct to an incorrect SMTP host and the sender address verification will fail.
  • If the account [email protected] does not exist, the sender verification will fail.
  • If you have disabled Allow empty sender address, the sender verification will fail, since the recipients SMTP server tries to verify by using an empty sender address.


These requirements were developed with consideration given to the standards developed by members of the global working group for counteracting spam, fraud and other forms of electronic abuse. 
Non-compliance with these requirements could lead to partial or full non-delivery of your email.

Technical requirements

  • All email must comply with RFC standards (for example, SMTP ; MIME , , , , ).
  • Bulk emails must be indicated by "Precedence: bulk" in the email headers.
  • All mail servers connecting to servers must have valid (real), meaningful, and not automatically generated reverse DNS records (rDNS, PTR). Additionally, the contact information for each IP address in WHOIS must be up-to-date and available. 

            Correct rDNS: 
            Incorrect rDNS:
  • All mail servers connecting to servers must be appropriately protected against unauthorized or anonymous use. Be certain that your server is not a public proxy server or a public relay.
  • Ensure that all web forms on your websites are safe.
  • If you use scripts that send emails from web forms, be sure that they cannot be used to send spam.
  • Direct connection to's MX servers from a dynamic IP address or home network address is not allowed.
  •'s MX servers must not be hard-coded in the sending servers' configuration files.
  • When using HTML in your messages, be sure that you form a valid HTML document. You cannot use potentially dangerous objects, such as ActiveX, JavaScript, VBScript, Java applets, frames or iframes, CSS connected to external websites, Meta Refresh, etc. Using these elements may lead to your distributions being blocked.
  • The use of external services (redirectors, link shortening services, etc.) in order to conceal information about the true destination webpage of any hyperlink in an email may lead to your distribution being blocked.
  • IP address and domain names may not be URL encoded in emails.

Administrative requirements

  • A distribution must only be sent upon the explicit and direct request or consent of a user (opt-in).
  • The text of every email in a subscription-based distribution must include the contact information of the organization sending the distribution, including a phone number and physical address.
  • Bulk distributions must have a simple and obvious way to unsubscribe. The unsubscription process must not require the user to perform complicated actions, such as entering or recovering a password, registering, etc. We recommend providing an unsubscription mechanism by including an easily noticeable link in each email in order to unsubscribe with a single click. Users must not be required to sign into a website in order to unsubscribe from a distribution.
  • Bulk emailers must not take actions to conceal, fabricate or distort a message's sender or delivery origin.
  • Information about the subscription, including how the email address was obtained, the subscription date and time, and the IP address from which the subscription was made, must be made available at the user's request.
  • Distributions must include information about the source of the user's email address, as well as their consent to receive the distrubution. For example, "You've received this message because you subscribed to this distribution on our website".
  • Services performing subscription-based distributions must unconditionally remove subscribers from their database or take measures to suspend distribution to them if their email addresses generate an SMTP error ( user not found). Tracking the validity of your user database is necessary towards maintaining a positive reputation as a bulk emailer.

User complaints, sending to nonexistent email addresses (including those that used to exist, but have since been deleted), the inability for the sender to receive failed delivery reports and being caught by's spam filters can all affect a sender's reputation. Any sender with a negative reputation may be blocked from sending messages to A series of violations from one bulk emailer could lead to an IP address being partially or fully blocked at

Error: Detailed Instructions for Fixing It

An SMTP session consists of back-and-forth communication between the client and the server. The client uses text commands and the server replies in numerical codes attached with messages. We discussed this communication in our blog post on SMTP Commands and Response Codes. Error codes can have different meanings on different servers, so you have several potential options for resolving the issue. Today we’ll discuss one of them, error, and explain how to fix it.

What does error mean?

The client typically receives a code as a reply to command. It is an error code that usually stands for some issue with the recipient server. The most frequent error message is “”. In most cases, this is because the recipient address does not exist. But there are other possible reasons and other possible error messages. 

error messages you can encounter


You may see this error message if the outgoing email was rejected. In this case, it is identifying an issue with a sending server that has used SMTP protocol instead of ESMTP. So, does this mean that error code can be caused by issues with both the recipient and sending server? Let’s find out.

Reasons for and solutions 

The basic reason for SMTP error is a typo in the recipient’s email address, such as a missed or duplicated character, resulting in an invalid address. So, email validation should be the first thing to verify. If the email address is valid, you will need to dig deeper.

Issues related to the receiving server

Email address does not exist

  • Check the email address for spelling errors.

MX record resolving to the wrong server

  • Check the routing and DNS resolution.
  • Contact the recipient server administrator to edit DNS records and set up correct MX records.
  • Use Telnet to connect to the receiving server.
  • If connection fails, the email may be blocked by the recipient server.
  • Explore possible issues on the sending server.

Issues related to the sending server

  • Using a dedicated tool, check to see if the IP address is blacklisted.
  • Examine email logs for spamming activity.
  • Check the server for malicious scripts.
  • Block any suspicious email account and strengthen password security.
  • Follow the blacklist-removal process.
  • Check DNS records including rDNS, SPF, DKIM, and DMARC.
  • Set up records that will ensure that IP addresses really belong to the domains they claim.

What might be wrong on the receiving end

Email address does not exist

The recipient server can’t find the email account to which you’ve sent an email. Therefore, the email will bounce back to the sender because it cannot be delivered.

How to solve this?

All you can do is recheck the email address for spelling errors. 

MX record resolving to the wrong server

A Mail Exchange (MX) record is a DNS record used by mail servers to determine where to deliver an email. The sender mail server looks for the MX record in order to communicate with the recipient mail server. The MX record points to the A record which, in turn, points to the mail server’s IP address. For example, the MX record of the domain points to the A record of, which points to If the MX record for the recipient domain resolves to some other server, you can get error. The email delivery fails because the receiving mail server cannot identify the email account in it. 

How to solve this?

First, you need to check the routing and DNS resolution using the tool with the parameter. For example:

The domain should resolve to the correct server on your end. After that, confirm the MX records for the domain using:

If you find any issues, contact the recipient server administrator to edit the DNS records and set up the correct MX records. The MX won’t resolve correctly if the DNS settings for the recipient’s domain are incorrect.

Custom security settings

It’s unlikely that a recipient server has no security mechanisms to protect from spam. If you get a error message, your email might have been rejected by spam filters. It does not mean that you’re a spammer or your email is spammy. Usually, strict security rules are in the way.

How to solve this?

Use telnet to connect to the receiving SMTP server (or its IP address) to check whether the firewall is blocking the sending server. We blogged about this in How to Test SMTP Server. Run the following:


If telnet says that you can’t connect, it means the recipient server has blocked your emails. So, the reason for the error is much closer to home – on the sending server.

What might be wrong on the sending end

Above we learned that the email sent from your server hit spam filters on the receiving end. There are a few reasons for that. 

Blacklisted IP

Here is how it looks. Some companies monitor servers and blacklist them if they are identified as spam senders. The majority of mail servers refer to these blacklists and reject emails coming from them. If you get error code, your server is likely on such a blacklist. Keep in mind that the use of a shared IP address increases the probability of being blacklisted. If you use a dedicated IP and are blacklisted anyway, your server was likely hacked.

How to solve this?

The first thing to be done is to check whether your IP is blacklisted. You can use such tools as IP Reputation Monitor by GlockApps, Blacklist Check by MXToolbox, or any other you like. After that, you need to follow the blacklist-removal process. Usually, it is based on the requirements of a particular blacklisting database.

Holders of dedicated IPs should examine their email logs for spamming activity. You also need to check the server for malicious scripts. After that, block the suspicious email account and strengthen your password policy. And don’t forget to change the password, of course.

Configuration issues

Another spam trigger refers to improper verification of the sending server. This refers to the verification of TXT DNS records (SPF, DKIM, and DMARC) and the reverse DNS record (rDNS). If you get a rejection, the DNS settings might be misconfigured.

How to solve this?

  • Perform a rDNS lookup manually with a command line using the command-line tool (in OSX and Linux) with the added flag lie as follows:

The full rDNS record will be shown in the “” of the outcome. The record must be set up and valid. You can also make use of online tools to verify rDNS:

Reverse DNS Lookup by

Reverse DNS Lookup by

Reverse Lookup by MXToolbox

  • Check the TXT records for your domain. This can be done manually using dig as follows:

and then 

is your name server, which you can find in the “” of the outcome to

  • Also, you can use as follows:

You’ll then need to find the SPF record that starts with . For example:

  • Update it according to your requirements. If the SPF record does not exist, make sure to create one. For more on this, read SPF Explained. 
  • Look for the DKIM record of your emails that starts with . You can validate the DKIM signature using an online tool like DKIM Record Check. For more on this, read DKIM Explained. 
  • Also, you need to look for the DMARC record that starts with . The receiving server always looks for it. If it is missing or is modified in any way, the whole verification will fail. For more on this, read DMARC Explained. 

All DNS records should be set up in a way to show that an IP address indeed belongs to the domain it claims. Otherwise, the recipient server may think that it is using someone else’s domain to send spam emails. You can read more about this in Why Emails Going to Spam and How to Prevent It. 

How to fix user unknown in Exchange, Exim, Postfix and Qmail servers

For Microsoft Exchange Servers

Security parameters are the main reason for error ‘’ in Exchange Servers and later. They authenticate the sender to prevent spamming and restrict email sending by default. All emails sent beyond local email addresses get bounced back with the error. Remove the restriction as follows:

  • Go to the Exchange admin center and navigate to Recipients > Mailboxes
  • Click the mailbox for which you want to set up mail forwarding
  • Click Mailbox Features on the mailbox properties page
  • Under Message Delivery Restrictions, select View details to view or change the delivery restrictions 
  • Clear the checkbox Require that all senders are authenticated. Click OK and then Save.

For Exim servers

For Exim mail servers, is not a common issue. But if it happens, the route likely needs to be identified. First, make sure that the mail server settings are correct. Then, verify filters and custom rules added to the server. This will help you identify and fix the .

For Postfix and Qmail servers

Start with the verification of the recipient domain resolution. It must resolve to the correct server. If the email service for the domain is hosted elsewhere, turn it off. Verify filters and custom rules added to the server. They must not mess up the mail delivery.

Can I avoid when sending bulk emails?

is a nightmare for any marketer who deals with bulk emails. Why? Just imagine that your email campaign has thousands of recipients. If you failed to verify all of them, it’s most likely that a large number of email addresses are invalid. This entails multiple bounces with . Will this have some sort of effect on email hosting providers? What do you think? Of course, it will, and your client will most likely be identified as a spammer sending emails to unverified addresses. 

You might have guessed that there is only one way to avoid this failure – keep your mailing lists clean. For this, you need to make sure that email address validation is a high-priority task. You can easily automate the process using bulk email verifiers available on the market. Some of them we’ve already introduced in the Email Testing Checklist. 

To wrap up

As you can see, is not an unsolvable issue. Be attentive to your server settings and do not neglect email address verification. By following these steps, you won’t jeopardize your email campaign. additionally, it’s advisable to test your emails for spam, and Mailtrap can help you with this. Mailtrap is a service for sending test emails via SMTP and also has features for email content verification and debugging. You can’t have too much of a good thing. For more on what else this service offers, read the Mailtrap Getting Started Guide.

Article byZakhar YungTechnical Content Writer @Mailtrap

If the basic variation of the SMTP error – “ requested action not taken mailbox unavailable” – is displayed, then the problem is usually that the intended target address couldn’t be reached by the SMTP server. In such cases, your first look should be at the address itself – it’s possible that a simple spelling error or typo slipped in when the e-mail was created. In many cases, though, the problem is on the recipient’s side. The message is typical if the recipient’s address isn’t temporarily available due to technical errors. The same generally goes for when the contacted e-mail account has been deactivated due to inactivity or deleted.

If you can reach the other party via another channel, you should do so. This way, you can make the problem known or find out the new, current (and valid) e-mail address. If you don’t have this option, then there’s nothing else to do other than try the sending again later. If the recipient was only temporarily unavailable, then the new attempt could be successful.

List of SMTP server return codes

This is a list of Simple Mail Transfer Protocol (SMTP) response status codes. Status codes are issued by a server in response to a client's request made to the server.

Unless otherwise stated, all status codes described here is part of the current SMTP standard, RFC&#; The message phrases shown are typical, but any human-readable alternative may be provided.

Basic status code[edit]

A "Basic Status Code" SMTP reply consists of a three digit number (transmitted as three numeric characters) followed by some text. The number is for use by automata (e.g., email clients) to determine what state to enter next; the text ("Text Part") is for the human user.

The first digit denotes whether the response is good, bad, or incomplete.:

  • 2yz (Positive Completion Reply): The requested action has been successfully completed.
  • 3yz (Positive Intermediate Reply): The command has been accepted, but the requested action is being held in abeyance, pending receipt of further information.
  • 4yz (Transient Negative Completion Reply): The command was not accepted, and the requested action did not occur. However, the error condition is temporary, and the action may be requested again.
  • 5yz (Permanent Negative Completion Reply): The command was not accepted and the requested action did not occur. The SMTP client SHOULD NOT repeat the exact request (in the same sequence).

The second digit encodes responses in specific categories:

  • x0z (Syntax): These replies refer to syntax errors, syntactically correct commands that do not fit any functional category, and unimplemented or superfluous commands.
  • x1z (Information): These are replies to requests for information.
  • x2z (Connections): These are replies referring to the transmission channel.
  • x3z&#;: Unspecified.
  • x4z&#;: Unspecified.
  • x5z (Mail system): These replies indicate the status of the receiver mail system.

Enhanced status code[edit]

The Basic Status Codes have been in SMTP from the beginning, with RFC&#; in , but were extended rather extensively, and haphazardly so that by RFC&#; rather grumpily noted that: "SMTP suffers some scars from history, most notably the unfortunate damage to the reply code extension mechanism by uncontrolled use."

RFC&#; defines a separate series of enhanced mail system status codes which is intended to be better structured, consisting of three numerical fields separated by ".", as follows:

class "." subject "." detail class = "2" / "4" / "5" subject = 1 to 3 digits detail = 1 to 3 digits

The classes are defined as follows:

  • Success: Report of a positive delivery action.
  • Persistent Transient Failure: Message as sent is valid, but persistence of some temporary conditions has caused abandonment or delay.
  • Permanent Failure: Not likely to be resolved by resending the message in current form.

In general the class identifier MUST match the first digit of the Basic Status Code to which it applies.[1]

The subjects are defined as follows:

  • XXXX Other or Undefined Status
  • XXXX Addressing Status
  • XXXX Mailbox Status
  • XXXX Mail System Status
  • XXXX Network and Routing Status
  • XXXX Mail Delivery Protocol Status
  • XXXX Message Content or Media Status
  • XXXX Security or Policy Status

The meaning of the "detail" field depends on the class and the subject, and are listed in RFC and RFC&#;

A server capable of replying with an Enhanced Status Code MUST preface (prepend) the Text Part of SMTP Server responses with the Enhanced Status Code followed by one or more spaces. For example, the " Bye" reply (after QUIT command) MUST be sent as " Bye" instead.[1]

The Internet Assigned Numbers Authority (IANA) maintains the official registry of these enhanced status codes.[2]

Common status codes[edit]

This section list some of the more commonly encountered SMTP Status Codes. This list is not exhaustive, and the actual text message (outside of the 3-field Enhanced Status Code) might be different.

&#; 2yz Positive completion[edit]

System status, or system help reply
Help message (A response to the HELP command)
<domain> Service ready
<domain> Service closing transmission channel
Authentication succeeded[3]
Requested mail action okay, completed
User not local; will forward
Cannot verify the user, but it will try to deliver the message anyway

&#; 3yz Positive intermediate[edit]

(Server challenge - the text part contains the Baseencoded challenge) [3]
Start mail input

&#; 4yz Transient negative completion[edit]

"Transient Negative" means the error condition is temporary, and the action may be requested again. The sender should return to the beginning of the command sequence (if any).

The accurate meaning of "transient" needs to be agreed upon between the two different sites (receiver- and sender-SMTP agents) must agree on the interpretation. Each reply in this category might have a different time value, but the SMTP client SHOULD try again.

Service not available, closing transmission channel (This may be a reply to any command if the service knows it must shut down)
A password transition is needed[3]
Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)
Requested action aborted: local error in processing
IMAP server unavailable[4]
Requested action not taken: insufficient system storage
Temporary authentication failure[3]
Server unable to accommodate parameters

&#; 5yz Permanent negative completion[edit]

The SMTP client SHOULD NOT repeat the exact request (in the same sequence). Even some "permanent" error conditions can be corrected, so the human user may want to direct the SMTP client to reinitiate the command sequence by direct action at some point in the future.

Syntax error, command unrecognized (This may include errors such as command line too long)
Authentication Exchange line is too long[3]
Syntax error in parameters or arguments
Cannot Basedecode Client responses[3]
Client initiated Authentication Exchange (only when the SASL mechanism specified that client does not begin the authentication exchange) [3]
Command not implemented
Bad sequence of commands
Command parameter is not implemented
Unrecognized authentication type[3]
Server does not accept mail[5]
Encryption Needed[6]
Authentication required[3]
Authentication mechanism is too weak[3]
Authentication credentials invalid[3]
Encryption required for requested authentication mechanism[3]
Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
User not local; please try <forward-path>
Requested mail action aborted: exceeded storage allocation
Requested action not taken: mailbox name not allowed
Transaction has failed (Or, in the case of a connection-opening response, "No SMTP service here")
Message too big for system[4]
Domain does not accept mail[5]


Below is an example SMTP connection, where a client "C" is sending to server "S":

S: ESMTP Postfix C: HELO S:, I am glad to meet you C: MAIL FROM:<[email protected]> S: Ok C: RCPT TO:<[email protected]> S: Ok C: RCPT TO:<[email protected]> S: Ok C: DATA S: End data with <CR><LF>.<CR><LF> C: From: "Bob Example" <[email protected]> C: To: Alice Example <[email protected]> C: Cc: [email protected] C: Date: Tue, 15 Jan C: Subject: Test message C: C: Hello Alice. C: This is a test message with 5 header fields and 4 lines in the message body. C: Your friend, C: Bob C: . S: Ok: queued as C: QUIT S: Bye {The server closes the connection}

And below is an example of an SMTP connection in which the SMTP Server supports the Enhanced Status Code, taken from RFC&#;

S: SMTP service ready C: EHLO S: says helloS: ENHANCEDSTATUSCODES C: MAIL FROM:<[email protected]> S: Originator <[email protected]> ok C: RCPT TO:<[email protected]> S: Recipient <[email protected]> ok C: RCPT TO:<[email protected]> S: Mailbox "nosuchuser" does not exist C: RCPT TO:<[email protected]> S: Forwarding to remote hosts disabledS: Select another host to act as your forwarder C: DATA S: Send message, ending in C: . S: Message accepted C: QUIT S: Goodbye {The server closes the connection}






The server is unable to connect.

Try to change the server&#;s name (maybe it was spelt incorrectly) or the connection port.

Connection refused or inability to open an SMTP stream.

This error normally refers to a connection issue with the remote SMTP server, depending on firewalls or misspelled domains. Double-check all the configurations and in case ask your provider.

System status message or help reply.

It comes with more information about the server.

A response to the HELP command.

It contains information about your particular server, normally pointing to a FAQ page.

The server is ready.

It&#;s just a welcome message. Just read it and be happy that everything is working (so far)!

The server is closing its transmission channel. It can come with side messages like &#;Goodbye&#; or &#;Closing connection&#;.

The mailing session is going to end, which simply means that all messages have been processed.

Its typical side message is &#;Requested mail action okay completed&#;: meaning that the server has transmitted a message.

The oppsite of an error: everything has worked and your email has been delivered.

&#;User not local will forward&#;: the recipient&#;s account is not on the present server, so it will be relayed to another.

It&#;s a normal transfer action. For other information check out our article on what is an SMTP server.

The server cannot verify the user, but it will try to deliver the message anyway.

The recipient&#;s email account is valid, but not verifiable. Normally the server relays the message to another one that will be able to check it.

The side message can be very cryptic (&#;Start mail input end <CRLF>.<CRLF>&#;). It&#;s the typical response to the DATA command.

The server has received the &#;From&#; and &#;To&#; details of the email, and is ready to get the body message.

&#;Timeout connection problem&#;: there have been issues during the message transfer.

This error message is produced only by GroupWise servers. Either your email has been blocked by the recipient&#;s firewall, or there&#;s a hardware problem. Check with your provider.

The service is unavailable due to a connection problem: it may refer to an exceeded limit of simultaneous connections, or a more general temporary problem.

The server (yours or the recipient&#;s) is not available at the moment, so the dispatch will be tried again later.

The recipient&#;s mailbox has exceeded its storage limit.

Best is to contact contact the user via another channel to alert him and ask to create some free room in his mailbox.

Not enough space on the disk, or an &#;out of memory&#; condition due to a file overload.

This error may depend on too many messages sent to a particular domain. You should try again sending smaller sets of emails instead of one big mail-out.

Typical side-message: &#;The recipient&#;s Exchange Server incoming mail queue has been stopped&#;.

It&#;s a Microsoft Exchange Server&#;s SMTP error code. You should contact it to get more information: generally it&#;s due to a connection problem.

The recipient&#;s server is not responding.

There&#;s an issue with the user&#;s incoming server: yours will try again to contact it.

The connection was dropped during the transmission.

A typical network connection problem, probably due to your router: check it immediately.

The maximum hop count was exceeded for the message: an internal loop has occurred.

Ask your SMTP provider to verify what has happened.

Your outgoing message timed out because of issues concerning the incoming server.

This happens generally when you exceeded your server&#;s limit of number of recipients for a message. Try to send it again segmenting the list in different parts.

A routing error.

Like error , it&#;s related only to Microsoft Exchange. Use WinRoute.

&#;Requested action not taken – The user&#;s mailbox is unavailable&#;. The mailbox has been corrupted or placed on an offline server, or your email hasn&#;t been accepted for IP problems or blacklisting.

The server will retry to mail the message again, after some time. Anyway, verify that is working on a reliable IP address.

&#;Requested action aborted – Local error in processing&#;. Your ISP&#;s server or the server that got a first relay from yours has encountered a connection problem.

It&#;s normally a transient error due to a message overload, but it can refer also to a rejection due to a remote antispam filter. If it keeps repeating, ask your SMTP provider to check the situation. (If you&#;re sending a large bulk email with a free one that can be a common issue).

Too many emails sent or too many recipients: more in general, a server storage limit exceeded.

Again, the typical cause is a message overload. Usually the next try will succeed: in case of problems on your server it will come with a side-message like &#;Out of memory&#;.

An error of your mail server, often due to an issue of the local anti-spam filter.

Contact your SMTP service provider to fix the situation.

A syntax error: the server couldn&#;t recognize the command.

It may be caused by a bad interaction of the server with your firewall or antivirus. Read carefully their instructions to solve it.

Another syntax error, not in the command but in its parameters or arguments.

In the majority of the times it&#;s due to an invalid email address, but it can also be associated with connection problems (and again, an issue concerning your antivirus settings).

The command is not implemented.

The command has not been activated yet on your own server. Contact your provider to know more about it.

The server has encountered a bad sequence of commands, or it requires an authentication.

In case of &#;bad sequence&#;, the server has pulled off its commands in a wrong order, usually because of a broken connection. If an authentication is needed, you should enter your username and password.

A command parameter is not implemented.

Like error , is a syntax problem; you should ask your provider.


Bad email address.

One of the addresses in your TO, CC or BBC line doesn&#;t exist. Check again your recipients&#; accounts and correct any possible misspelling.

A DNS error: the host server for the recipient&#;s domain name cannot be found.

Check again all your recipients&#; addresses: there will likely be an error in a domain name (like [email protected] instead of [email protected]).

&#;Address type is incorrect&#;: another problem concerning address misspelling. In few cases, however, it&#;s related to an authentication issue.

Doublecheck your recipients&#; addresses and correct any mistake. If everything&#;s ok and the error persists, then it&#;s caused by a configuration issue (simply, the server needs an authentication).

The total size of your mailing exceeds the recipient server&#;s limits.

Re-send your message splitting the list in smaller subsets.

Normally, an authentication problem. But sometimes it&#;s about the recipient&#;s server blacklisting yours, or an invalid email address.

Configure your settings providing a username+password authentication. If the error persists, check all your recipients&#; addresses and if you&#;ve been blacklisted.

The recipient address rejected your message: normally, it&#;s an error caused by an anti-spam filter.

Your message has been detected and labeled as spam. You must ask the recipient to whitelist you.

It usually defines a non-existent email address on the remote side.

Though it can be returned also by the recipient&#;s firewall (or when the incoming server is down), the great majority of errors simply tell that the recipient email address doesn&#;t exist. You should contact the recipient otherwise and get the right address.

&#;User not local or invalid address – Relay denied&#;. Meaning, if both your address and the recipient&#;s are not locally hosted by the server, a relay can be interrupted.

It&#;s a (not very clever) strategy to prevent spamming. You should contact your ISP and ask them to allow you as a certified sender. Of course, with a professional SMTP provider like turboSMTP you won&#;t ever deal with this issue.

&#;Requested mail actions aborted – Exceeded storage allocation&#;: simply put, the recipient&#;s mailbox has exceeded its limits.

Try to send a lighter message: that usually happens when you dispatch emails with big attachments, so check them first.

&#;Requested action not taken – Mailbox name invalid&#;. That is, there&#;s an incorrect email address into the recipients line.

Check all the addresses in the TO, CC and BCC field. There should be an error or a misspelling somewhere.

This means that the transaction has failed. It&#;s a permanent error and the server will not try to send the message again.

The incoming server thinks that your email is spam, or your IP has been blacklisted. Check carefully if you ended up in some spam lists, or rely on a professional SMTP service like turboSMTP that will nullify this problem.


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