Anybody who sends email has seen them, in one form or another - those SMTP error codes, often returned in bounced email, such as "550 Requested action not taken: mailbox unavailable" or "550 5 2 1 mail from refused spam site." These are often 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? And why should you care? ('SMTP' stands for Simple Mail Transfer Protocol.) First, here is a list of SMTP messages and error messages, and what each message is supposed to say and mean
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