You are reading this because you’ve received an automated message with a “550 5.1.1” error code in it. There are couple of things that could have caused this error:

• The recipient address does not exist: you may possess an outdated email that was valid at one time.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• If your MX record was changed, you may have an outdated DNS record and you were trying to resolve your MX record to an IP that no longer hosted it.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