Outbound Mail Delivery is failing with Error code " 550 5.1.1 User Unknown "

Products

• Email Security.cloud

Issue/Introduction

The Mail delivery to the recipient server fails with the Error code " 550 5.1.1 User unknown "

 

Environment

Email Security. cloud

Cause

1. The 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.
2. 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 up an email address.
3. 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.
4. 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.
5. 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 ‘.

Resolution

Please reach out to the recipient's mail server administrator to resolve the delivery issue related to the intended recipient's email address.