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 codes and error codes, and what each message is supposed to say and mean:
211 - A system status message.
214 - A help message for a human reader follows.
220 - SMTP Service ready.
221 - Service closing.
250 - Requested action taken and completed.
251 - The recipient is not local to the server, but the server will accept and forward the message.
252 - The recipient cannot be VRFYed, but the server accepts the message and attempts delivery.
354 - Start message input and end with .. This indicates that the server is ready to accept the message itself
421 - The service is not available and the connection will be closed.
450 - The requested command failed because the user's mailbox was unavailable (such as being full). Try again later.
451 - The command has been aborted due to a server error. (on their side)
452 - The command has been aborted because the server has insufficient system storage.
500 - The server could not recognize the command due to a syntax error.
501 - A syntax error was encountered in command arguments.
502 - This command is not implemented.
503 - The server has encountered a bad sequence of commands.
504 - A command parameter is not implemented.
550 - The requested command failed because the user's mailbox was unavailable (such as not found)
551 - The recipient is not local to the server.
552 - The action was aborted due to exceeded storage allocation.
553 - The command was aborted because the mailbox name is invalid.
554 - The transaction failed for some unstated reason.
