Where it was working before and then stopped, I Would suspect a change on the server, maybe the server has changed the SMTP server address.
to help troubleshoot if it is the code sort the server, edit the verify page, change the redirect:
$GoToPage = "thank-you.html";
to be blank:
$GoToPage = "";
then add the following code at the end to write the session contents to the page:
<?php @session_start(); var_dump($_SESSION); ?>
the session contents will contain the status of passing the email to the SMTP server.
If the status says success you will need to contact the host to determine why the SMTP server is not relaying the email.