‘550 5.1.1 User unknown’ is one of the common issues reported by users sending emails to Exchange servers. 550 error means there is some problem with the recipient server which makes it unable to deliver the mail.for the more information read the below Article…
- The recipient address does not exist: you may possess an outdated email that was valid at one time.
- There are typos in the recipient’s email address, which means that the address to which you are trying to send the email has never existed.
- If your MX record was changed, you may have an outdated DNS record and you were trying to resolve your MX record to an IP that no longer hosted it.
- There is something wrong with the configuration of the receiving server.
The most common 550 5.5.1 messages-
- 550 5.1.1: The email account that you tried to reach does not exist
- 550 5.1.1: Recipient rejected
- 550 5.1.1: User unknown
- 550 5.1.1: Recipient address rejected: User unknown in virtual alias table
Ways you can fix the 550 5.1.1 error-
- Check the email for typos (sometimes they are quite obvious, for example:
email@example.com) and verify it. You can use an online tool that serves this purpose—there are lots of them on the Internet.
- Your DNS record is stale and cannot be resolved by the recipient server. It needs to be updated. Perform DNS lookup for your domain (DNS Lookup) and push the “Find problems” button. Consult the webmaster if you see any errors in the resulting list.
- If everything looks okay, or you get the same error when trying to send the email again, even though you are sure that the address exists and is typed correctly, you should find another way to contact the owner of the email and let them know that there is something wrong with the mail server configuration on their side—there is nothing you can do about it yourself.