Warning: mail() [function.mail]: SMTP server response: 501 5.5.4 Invalid Address

I'm having the following error after I sign up for a new account on the Million Pixel page I made... New user created test test, You have successfully signed up to the Magic squares Advertiser's System. You will soon receive a validation email to verify your email address. If you ever encounter any problems, bugs or just have any questions or suggestions, feel free to use contact [redacted email] Warning: mail() [function.mail]: SMTP server response: 501 5.5.4 Invalid Address in mpixelincludefunctions.php on line 1451 Warning: mail() [function.mail]: SMTP server response: 501 5.5.4 Invalid Address in mpixelincludefunctions.php on line 1451 Does anyone know how to fix this?