- 500 – The server could not recognize the command due to a syntax error.
- 501 – A syntax error was encountered in command arguments.
- 502 – This command is not implemented.
- 503 – The server has encountered a bad sequence of commands.
- 504 – A command parameter is not implemented.
- 550 – No mailbox by that name is currently available (for example because it was not found, or because the command was rejected for policy reasons including a full mailbox.
- 551 – The recipient is not local to the server. The server then gives a forward address to try.
- 552 – The action was aborted due to exceeded storage allocation.
- 553 – The command was aborted because the mailbox name is invalid.
- 554 – The transaction failed. Blame it on the weather.