does it redirect to the thank you page?
if it directs to the thank you page, then the email is being sent to the SMTP Server, it is the SMTP server that is refusing to relay the message.
if it does not redirect to the thank page, then there may be a php error occurring.
add the following cade to line 1 of the page to turn error reporting on:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>