I'm not able to access the form to investigate.
When emails are not sending, here are a few things that can help diagnose the problem:
1) add the following cade at line 1 to turn on error reporting:
2) Add the following code to output the session details on the success page just before the </body> tag:
<pre><?php var_dump($_SESSION); ?></pre>
this will include information about the email including whether it was passed to the SMTP server.
It's tough to tell why the email is not sending without being able to investigate.