If you receive the following error when attempting to send email through your server, you need to enable SMTP Auth.

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was ' This e-mail address is being protected from spam bots, you need JavaScript enabled to view it '. Subject 'Subject', Account: 'mail.yourdomain.com', Server: 'mail.yourdomain.com', Protocol: SMTP, Server Response: '550 5.7.1 ... Relaying denied', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

This error message appears a result of security settings to prevent spammers from using your server to send email. To be able to send SMTP emails, you must enable SMTP Authentication.

For Outlook Express:

Tools > Accounts
Click on the Mail tab
Select the account you are trying to send email through
Select Properties
Click on the Servers tab
Click the check box on "My server requires authentication"