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. The best message of them all.
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 <CRLF>.<CRLF>. This indicates that the server is ready to accept the message itself (after you have told it who it is from and where you want to to go).
421 – The service is not available and the connection will be closed.
450 – The requested command failed because the user’s mailbox was unavailable (for example because it was locked). Try again later.
451 – The command has been aborted due to a server error. Not your fault. Maybe let the admin know.
452 – The command has been aborted because the server has insufficient system storage.
The following error messages (500-504) usually tell you that your email client is broken. It’s probably best to let the program’s author know.
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 (for example because it was not found, or because the command was rejected for policy reasons).
551 – The recipient is not local to the server. The server then gives a forward address to try.
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. Blame it on the weather.