i looked at the /contact-us.php page and can see it is redirecting to the thank you page.
To troubleshoot, remove the redirect and add the following code in the body:
<?php @session_start(); var_dump($_SESSION); ?>
that will write the session contents to the page, including the status of passing the email to the SMTPO Server.
If that status returns Success, then the SMTP Server is refusing to relay the email for some reason, and you will need to contact the host to determine why.