Error using PHP Contact Form

I tested the form from this site on my GoDaddy site. I got the message “Data has been submitted to …” ok, but that was followed by this error:
Warning: mail() [function.mail]: SMTP server response: 451 See http://pobox.com/~djb/docs/smtplf.html. in D:\Hosting\xxxxxx\html est\mailer.php on line **23.

**The test message did not get sent (I never received it, and I did set my real email address into the code).

The URL link goes to a page that talks about ‘bare line-feeds in mailing’ problems - as far as I can see there are none in the script.

Line 23 is: mail($to, $subject, $body);
though the actual cause of the error is probably elsewhere.

Does anyone have any idea what is going on here?