Hoping someone can help me

I am installing zencart, first time on a windows host. The http is fine, but having trouble with the https which is windows (assuming thats my problem)
Everything else seems to be ok but when it needs to send mail it returns this error
Warning: mail() [function.mail]: SMTP server response: 501 5.5.4 Invalid Address in d:\websites\ssl\888888888\store\includes\classes\email.php on line 520

Warning: Cannot modify header information - headers already sent by (output started at d:\websites\ssl\888888888\store\includes\classes\email.php:520) in d:\websites\ssl\88888888888\store\includes\functions\functions_general.php on line 52
Any help would be greatly appreciated.