What does the SMTP response code 550 indicate?

The SMTP response code 550 indicates that the requested action was not taken because the mailbox is unavailable. This code is typically returned when an email is sent to a recipient whose mailbox does not exist, is not accessible, or is otherwise unavailable. Essentially, it signifies that the server cannot deliver the email because it cannot find the recipient's address in its system.

When a sender receives a 550 response, it is crucial to check the recipient's email address for accuracy, ensure that the mailbox is active, and confirm that the recipient's email server is functioning correctly. This information helps the sender understand that the problem lies with the recipient's mailbox rather than an error in sending or transmission.