211 System status, or system help reply.
214 Help message.
220 Domain service ready.Ready to start TLS.
221 Domain service closing transmission channel.
250 OK, queuing for node node started.Requested mail action okay, completed.
251 OK, no messages waiting for node node.User not local, will forward to forwardpath.
252 OK, pending messages for node node started.
Cannot VRFY user (e.g., info is not local), but will take message for this user and attempt delivery.
253 OK, messages pending messages for node node started.
354 Start mail input; end with <CRLF>.<CRLF>.
355 Octet-offset is the transaction offset.
421 Domain service not available, closing transmission channel.
432 A password transition is needed.
450 Requested mail action not taken: mailbox unavailable.ATRN request refused.
451 Requested action aborted: local error in processing.Unable to process ATRN request now
452 Requested action not taken: insufficient system storage.
453 You have no mail.
454 TLS not available due to temporary reason.Encryption required for requested authentication mechanism.
458 Unable to queue messages for node node.
459 Node node not allowed: reason.
500 Command not recognized: command.Syntax error.
501 Syntax error, no parameters allowed.
502 Command not implemented.
503 Bad sequence of commands.
504 Command parameter not implemented.
521 Machine does not accept mail.
530 Must issue a STARTTLS command first.Encryption required for requested authentication mechanism.
534 Authentication mechanism is too weak.
538 Encryption required for requested authentication mechanism.
550 Requested action not taken: mailbox unavailable.
551 User not local; please try forwardpath.
552 Requested mail action aborted: exceeded storage allocation.
553 Requested action not taken: mailbox name not allowed.
554 Transaction failed.