SMTP and Mail server error codes

Short description of SMTP/ESMTP error codes and error messages

How often do you get "FATAL ERROR: Could not send your message. Error #550" when sending email? What does "Error #550" mean? Ordinary users may only understand what their e-mail was not sent. Why and what to do?

You need to know the structure of these error codes. A mail server will reply to every request a client (such as your email program, like Outlook Express, Eudora, Atomic Mail Sender) makes with a return code. This code consists of three numbers. First digit tells whether the server accepted the command and if it could handle it. There are five possible values:

1. Mail server has accepted the command, but does not yet take any action. A confirmation message is required.
2. Mail server has completed the task successfully without errors.
3. Mail server has understood the request, but requires further information to complete it.
4. Mail server has encountered a temporary failure. If the command is repeated without any change, it might be completed. Try again, it may help!
5. Mail server has encountered a fatal error. Your request can't be processed.

Second digit tells more. There are six possible values:

0. Syntax error
1. Information reply (for example to HELP request)
2. This digit refers to the status of connection
3. non-used
4. non-used
5. This digit refers to the status of the mail server

The third digit tells more then second again. It shows more graduations of the mail transferring status. Combination of these three digits gives us detailed list of ESMTP server response codes, as layed down in RFC 821 and later extensions.