These error codes are drawn from RFC 821 (RFC 821 is from August 1982, isn't that neat?)
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: 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
