XMail. Understanding SMTP Error Messages. Way too often, error messages are but mystic numbers forming some kind of incomprehensible code. This page wants to be your guide to the code mail servers produce when you try to send mail, but it doesn't work. "Could not send your message. Error 421. " This was all the email client showed her. No, it is not OK. It is not okay, but only too often it happens. What's your next step? The Meaning of the Numbers A mail server will reply to every request a client (such as your email program) makes with a return code. The first generally tells whether the server accepted the command and if it could handle it. 1: The server has accepted the command, but does not yet take action. The second number gives more information. 0: A syntax error has occurred. 1: Indicates a informational reply, for example to a HELP request. 2: Refers to the connection status. 3 and 4 are unspecified. 5: Refers to the status of the mail system as a whole and the mail server in particular.
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