SMTP Codes

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