- 500 - The server could not recognize the command due to a syntax error.
- 501 - A syntax error was encountered in command parameters or 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 (ex. because it was not found, or because the command was rejected due to policy reasons, such as a full mailbox. Please clear the callout cache after the mailbox has been emptied).
- 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.
Possible 5xx codes
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