mpform message never arrives/email not sent

[pixability]

Hi, I've implemented the latest version of mprfom into a client's website and somehow the filled in content never arrives.
<?php

if (mail("test@test.com.au","Hello","This is a test")) {
  echo "Success";
} else {
  echo "Failure";
}

?>

echos "Success", so the mail function itselt is working.
Also, the message "Thank you for submitting your form" is displayed on the following page and a confirmation email is sent to the sender. Its just that the actual message disappears in nowhereland.
Can someone give me a hint how to bugfix this?

Thanks!