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> 

345 

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