What are the SMTP Error Codes and what do they mean?

Possible 5xx codes

• 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. 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.