SMTP codes can be so confusing, especially those SMTP response codes and SMTP error codes like the dreaded SMTP error 550. These are often returned in bounced email, such as “550 Requested action not taken: mailbox unavailable” or “550 5 2 1 mail from refused spam site.” They are in response to SMTP commands that have ‘gone wrong’ between your email server that sent the email, and the receiving email server that is unable to deliver it (or refuses to deliver it) for some reason. But what exactly do they mean? Why should you care and what should you do? And why do we say that SMTP error 550 is dreaded?

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