When you get an error message as the one stated below, you may need to check if SMTP Authentication is enabled in your mail client
"The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'firstname.lastname@example.org'.Subject 'Read: Test Subject', Account: 'mail.yourdomain.com', Server:'mail.yourdomain.com', Protocol: SMTP, Server Response: '503 This mail server requires authentication when attempting to send to anon-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.', Port: 25, Secure(SSL): No, Server Error:503, Error Number: 0x800CCC79"?
503 valid RCPT command must precede DATA
530 Authentication required
550 Access denied - Invalid HELO name (See RFC2822.214.171.124)
If you received the error messages as above when you are trying to send mail, then you may need to configure your email client (ie Outlook Express, Outlook) to authenticate automatically by checking the "My server requires authentication" box in your email client configuration.
You may refer below links on the how to enable SMTP authentication
How to enable SMTP authentication for Outlook Express
How to enable SMTP authentication for Microsoft Outlook
How to enable SMTP authentication for Windows Mail, Netscape, Eudora,Opera, Mozilla Thunderbird and Entourage: