with this info, all you should need to do is add the ini set line at line 1 of the contact us page:
<?php
ini_set("sendmail_from", "user@yourdomain.com");
?>
then edit the WA_UniversalEmail/mail_php.php file.
change line 257:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader);
to:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader, "-fuser@yourdomain.com");
in both of these code examples, make sure to change user@yourdomain.com to a valid email account on your fasthosts domain.