SMTP reply codes

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

211 System status, or system help reply
214 Help message [Information on how to use the receiver or the meaning of a particular on-standard command; this reply is useful only to the human user]

220 Service ready
221 Service closing transmission channel
421 Service not available, closing transmission channel [This may be a reply to any command if the service knows it must shut down]

250 Requested mail action okay, completed
251 User not local; will forward to
450 Requested mail action not taken: mailbox unavailable [E.g., mailbox busy]
550 Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
451 Requested action aborted: error in processing
551 User not local; please try
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect]
354 Start mail input; end with .
554 Transaction failed

Source: RFC 821