5yz Permanent Negative Completion

The command was not accepted and the requested action did not occur. The SMTP client SHOULD NOT repeat the exact request (in the same sequence). Even some “permanent” error conditions can be corrected, so the human user may want to direct the SMTP client to reinitiate the command sequence by direct action at some point in the future (e.g., after the spelling has been changed, or the user has altered the account status).

500
Syntax error, command unrecognized
501
Syntax error in parameters or arguments
502
Command not implemented
503
Bad sequence of commands
504
Command parameter not implemented
521
Server does not accept mail Source
550
Requested action not taken: mailbox unavailable
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
554
Transaction failed
555
MAIL FROM/RCPT TO parameters not recognized or not implemented
556
Domain does not accept mail Source