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 non-standard command; this reply is useful only to the human user] 220 <domain> Service ready 221 <domain> Service closing transmission channel 421 <domain> 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 <forward-path> 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 <forward-path> 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 <CRLF>.<CRLF> 554 Transaction failed