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