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;

                 his 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

    552     Requested mail action aborted: exceeded storage allocation

    553     Requested action not taken: mailbox name not allowed