SMTP codes from RFC 821 RFC 821 SMTP codes The following are SMTP codes sent by a reporting MTA or Mail Transport Agent.

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 domain name Service ready

221 domain name Service closing transmission channel

250 Requested mail action okay, completed

251 User not local; will forward to 
          
354 Start mail input; end with .
          
421 domainname 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