What does the 503 error mean?
Our servers need to authenticate you as a legitimate user before allowing you to send email out.
The simplest way you are authenticated is when you’ve successfully checked your email. Successfully checking your email means you have access to the correct username and password, and are therefore a legitimate user.
The “503 valid RCPT command must precede DATA” error message is shown when our servers have not been able to authenticate you prior to your attempt to send email.
How do I fix the problem?
Configure your email software to authenticate you by ticking the “My server requires authentication” option or similar. The server will then authenticate your username and password each time you send mail.