who is your host?
on the thank-you.php page, add the following code:
<?php @session_start(); var_dump($_SESSION); ?>
this will write the session contents to the page which will contain details about the status email being sent to the SMTP server. This will let you know if the message is being sent to the SMTP server or not. if it is making it to the SMTP Server, there is something on the SMTP server that is causing the message not to be relayed. you would need to access the SMTPO server error logs to find out what that is.