‘550 5.1.1 User unknown’ – How to fix this email error in Exchange, Postfix and Qmail servers


‘550 5.1.1 User unknown’ is one of the common issues reported by users sending mails to Exchange servers. 550 error means there is some problem with the recipient server which makes it unable to deliver the mail.

In our role as Website Support specialists for web hosting companies, we’ve seen senders sending mails from other servers such as Exim, Postfix, etc. to Exchange servers, seeing this error in the bounce message:

550 5.1.1 User unknown (in reply to RCPT TO command)

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Today we’ll see the major causes of this email error and how to fix them.

1. Wrong recipient address

Sender would get the bounce message ‘550 5.1.1 User unknown‘, if the recipient mail server is unable to find the recipient mail account in it.

At times, a typo in the email address of the recipient can cause this email delivery failure. A missing alphabet or an incorrect character can end up messing an email address.

It is possible that an email address that once existed, gets deleted or moved from the recipient mail server. In such cases, the recipient server would be unable to deliver the mail.

Even a misconfiguration or bug in the recipient mail server can cause the email account to get disabled and mails to that account to fail.

If the recipient doesn’t have the email address that you’re sending to, it will bounce the mail with the error message ‘550 5.1.1 User unknown‘.

Solution:

Always validate the recipient email address you’ve given in the email for any spelling mistakes or errors. If everything is fine at your end, confirm whether the email account actually exists.

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2. Incorrect DNS settings for the domain

Email Error 550 can also occur when the DNS settings for the recipient domain are not correct or the MX entry is not resolving correctly.

For instance, the MX records for the domain could be wrongly resolving to the recipient mail server instead of its intended server.

In such cases, recipient mail server cannot find the account and will bounce the mail. DNS issues can also occur due to any wild card DNS or SPF entry that is not set correctly.

These DNS related errors or wrongly mail server MX settings can end up giving an error message ‘550 5.1.1 User unknown‘ to the sender.

Solution:

Verify the MX records for a domain, using the command:

dig domain.com MX

If you notice DNS errors or missing MX records, then contact the recipient server support and get it fixed.

3. Email filters set for the domain

Some recipient mail accounts can have email filters or forwarders set. A catchall address or forwarder that relay to a non-existing account, can end up causing 550 errors.

Filtering for incoming mails using email filters or anti-spam firewalls, can end up bouncing emails from certain sender servers, returning a 550 error message.

Though 550 errors are usually related to recipient mail server issues, at times some filtering or firewalls applied on outgoing mails in the sender server can also cause this error.

For instance, some programs such as Google Apps modify the ‘To address’ of outgoing mails. Updating the recipient address to a non-existing address, can show error ‘550 5.1.1 User unknown‘.

Solution:

Check your outgoing firewalls and rules, if any. If all good, contact recipient server support and get the email filters verified for any erroneous filters or rules that can be blocking mails.

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4. Mail server configuration errors

Apart from the above reasons, some configuration mistakes in the mail server can also end up causing the error 550 for domains in it.

A server-wide custom filter that blocks mail from certain senders, a default forwarder that relay mails to another address, custom email software or security restrictions, all can lead to email errors.

Sometimes, the issue may be a temporary one. In those cases, the sender mail server will re-attempt the delivery of failed emails, based on its configuration.

Solution:

Examining the error logs and email headers of the bounced mail would help identify the issue with the mail server. Testing email routing in the server would also help detect any issues.

Disabling unwanted filters and rules and ruling out any restrictions that cause the emails to bounce, is crucial in fixing the issue.



How to fix the error ‘550 5.1.1 User unknown’ in Exchange servers

The error ‘550 5.1.1 User unknown‘ is commonly noticed in Microsoft Exchange servers. It can happen either with a single mail account or a distribution group of email addresses.

Microsoft Exchange 2010 has implemented a security parameter which require that all senders are authenticated, to prevent spamming. By default, this feature is enabled in the server.

As a result, only local email addresses would be able to send mails to these addresses. All mails from external addresses gets bounced with 550 error.

The solution is to check and verify that there are no security or restrictions with these accounts that are causing the bounce and email error 550.

To remove the restriction on an email account or a distribution list, do these steps:

  1. Open the Exchange management console.
  2. Expand recipient configuration and click on mailbox or list.
  3. Select the mailbox or list that is having the issue receiving the mail.
  4. Click on Properties -> ‘Mail Flow Settings’ tab.
  5. Click the ‘Message Delivery Restrictions’ and click properties.
  6. Clear the check-box for ‘Require that all senders are authenticated’ and click OK.

How to fix the error ‘550 5.1.1 User unknown’ in Exim for cPanel/WHM and DirectAdmin servers

Most cPanel and DirectAdmin servers use Exim as their mail server. While some choose the default installation, some others go for further customization of Exim.

The error message ‘550 5.1.1 User unknown’ is not typical to Exim mail server. So if your Exim server is giving out this message, it could be due to some other software integrated with it.

Check the recipient email address for typo errors and whether it exists in the server. Also confirm that there are no custom email programs that are messing up the mail server settings.

Verifying the email filters and custom rules added to the Exim mail server or specific to that domain, helps to identify and fix the 550 email errors.

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How to fix the error ‘550 5.1.1 User unknown’ in Qmail and Postfix for Plesk servers

First of all, verify that the recipient domain is properly resolving to the correct server and it has its mail server configured in this server.

If the MX points to the local server, check if the email account is disabled, and if so, re-enable it. If the email service for the domain is hosted elsewhere, just turn off mail services for that domain.

Confirm that there are no custom rules or filters in the Qmail or Postfix mail server that is messing up with the mail delivery. A detailed examination of bounced headers and mail logs would show that.

To add on..

There are a few other variants for this error ‘550 5.1.1 User unknown‘, varying with the custom message configured in each mail server:

550 5.1.1 user unknown

550 MAILBOX NOT FOUND

550 unrouteable address

550 mailbox temporarily disabled

550 No such user here

550 Requested action not taken: mailbox unavailable

550 : Recipient address rejected: User unknown in local recipient table

Though the message shown varies, the reason and fix for these errors are same as the ones we’ve already discussed here.

There are other 550 email errors too, that are due to other reasons such as lack of SMTP authentication or improper relaying.

Bobcares helps cPanel webmasters quickly resolve email errors, thereby minimizing business downtime. If you’d like to know how to fix your mail server, we’d be happy to talk to you.


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