It looks like you are using fasthosts as you hosting provider?
I found the following on their site:
it looks like they have a couple of requirements to lock down the smtp server.
<?php ini_set("sendmail_from", "email@example.com"); ?>
to line 1 of the registration page.
Then in the WA_UniversalEmail/mail_PHP.php file, find the following:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader);
and change it to:
$mailObj = mail($mailTo,$mailSubject,$mailContent,$mailHeader, "-firstname.lastname@example.org");