Get mail() [fuction.mail] error - need help fixing

Warning: mail() [function.mail]: SMTP server response: 550 5.7.1 ....D:\sites\engineering\about\mail.php on line 405

I get this message as a result on submitting a form.
What should I look for?
What could the problem be?
Is it a simple fix?
Please send me some guidance or advice.
Thanks,
Vicky