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 will accept and forward this message.
252 – The recipient cannot be verified, but will accept the message and attempt delivery.
421 – The service is not available and the connection will be closed.
450 – The requested command failed because the user’s mailbox was unavailable. So try again later.
451 – The command has been aborted due to a server error. Not your problem.
452 – The command has been aborted because the server has insufficient system storage.
500 – The server could not recognise 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.
551 – The recipient is not local to the server. The server might give an address to try.
552 – The action was aborted because of a storage allocation limit.
553 – The command was aborted because the mailbox name is invalid.
554 – The general purpose ‘transaction failed’