The script your host is using, adds the -f option to the mail() function call.
you can add this to the user registration solution pack by modifying the WA_Universal_Email/mail_PHP.php file. Find the following code:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader);
and change it to:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader, "-f".$mailFrom);
for the security assist pages, make the same code change to the WA_SecurityAssist/Mail_PHP.php file.