.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> Service ready.
.221
<domain> Service closing transmission channel.
.250
Requested mail action okay, completed.
.251
User not local; will forward to <forward-path>.
          
.354
Start mail input; end with <CRLF>.<CRLF>.
          
.421
<domain> 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 <forward-path>.
.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.
