200 - A non-standard response reference RFC876
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. 
253 - Messages for "node" have started.

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.
432 - A password transition is needed.
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. 
453 - You have no mail
454 - TLS not available due to temporary reason: Encryption required.
458 - Unable to queue messages for node.
459 - Node not allowed: "reason"

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. 
505 - Authentication required.
521 - Machine does not accept mail.
530 - Must issue STARTTLS command first: Encryption required.
534 - Authentication mechanism too weak.
538 - Encryption required for requested authentication.
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.
571 - No external routing allowed.
572 - Bounces are blocked.