The most common type of “550 5.5.1” error messages are:
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
Major causes of “550 5.5.1” error messages are:
Wrong recipient address
Incorrect DNS settings for the domain
Email filters set for the domain
Mail server configuration errors
Solution to fix “550 5.5.1” error:
It’s recommended to validate the recipient email address which you’ve given in the email for any spelling mistakes or errors. That means you need to check the email for typos sometimes which is quite obvious, for example: email@example.com / firstname.lastname@example.org and verify it.
Execute a DNS lookup for your domain and press the “Find problems” button. If you see any DNS errors or missing MX records, then contact the webmaster/recipient server support and get it fixed.
If you have any outgoing rules/filters and firewall set then double check. If all looks good, then you have to contact recipient server support team and get the email filters verified for any erroneous filters or rules that can be blocking mails.
If everything looks okay, then you need to find another way to contact the owner of email and let them know that there is something wrong with the mailserver configuration on their side. Although, you can check the error logs and email headers of the bounced mail which would help identify the issue with the mail server.