Hi...I have got a wired problem here....please help me if you can....

I am receiving following error on my site after filling the registration form for oscommerce...
Please help on how to get rid of the error......

Warning: mail(): SMTP server response: 501 Invalid Address in C:\Inetpub\vhosts\bestsoftstore.com\httpdocs\oscommerce\includes\classes\email.php on line 502

Warning: Cannot modify header information - headers already sent by (output started at C:\Inetpub\vhosts\bestsoftstore.com\httpdocs\oscommerce\includes\classes\email.php:502) in C:\Inetpub\vhosts\bestsoftstore.com\httpdocs\oscommerce\includes\functions\general.php on line 29

eventhough it is giving the above error it is registering the customer details.it is updating the database....so i guess only it is not sending the email replies of form submission to recipient......i don't know what to do now....
I am using windows hosting.please let me know for a windows hosting what i can do to get the registration form working...

thanks in advance,,
Ked