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

‘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‘.


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.


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

dig 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‘.


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 such as the SSL settings or the outgoing SMTP settings, 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.


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.

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  1. I have this problem from time to temporary for some hours. Sometimes the mails finds its destination some it bounces. When I look at the firewall log I cann see that the Mail doesnt even get to the Firewall when it bounces back

    • Stefan,

      Feel free to contact our expert server administrators at if the issue persists. We’ll check your server in detail and resolve this for you in no time.


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