ESMTP Status/Error CodesA typical SMTP response looks like:
214 2.0.0 End of HELP info
Where
214 is the new enhanced SMTP (ESMTP) status code
2.0.0 is the old style SMTP status code (they remain for backwards compatibility with old mail servers)
The point of the new ESMTP codes is to return a more detailed status code. The new ESMTP codes build on the old SMTP codes (they still use the major classes).
The major SMTP status classes/codes (first digit) are:
2.x.x Success
4.x.x Persistent Transient Failure
5.x.x Permanent Failure
These explanations are mostly from savemail.c
ESMTP STATUS CODE DESCRIPTION
211 System status, or system help reply
214 Help message
(Information on how to use the receiver or the meaning of a particular non-standard command; this reply is useful only to the human user)
220 Service ready
221 Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to
354 Start mail input; end with "."
421 Service not available, closing transmission channel
(This may be a reply to any command if the service knows it must shut down)
450 Requested mail action not taken: mailbox unavailable
(E.g., mailbox busy)
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage
500 Syntax error, command unrecognized
(This may include errors such as command line too long)
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
(E.g., mailbox not found, no access)
551 User not local; please try <....>
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
(E.g., mailbox syntax incorrect)
554 Transaction failed
