Do you have Universal Email to redirect to a Success page after sending the email?
If so, does the redirect work?
on the success page, try adding the following code in the body:
<pre><?php @session_start(); var_dump($_SESSION); ?></pre>
this will write the session contents to the page. including in the session contents will be a status message on sending the email to the SMTP server, does the status return success or fall?
if it returns success, that means UE is compiling the email message and has passed it to the SMTP Server. so that will mean the problem is on the server end and you will need to contact the host for information on why it is not replying to those suffixes.
if it returns failure, I'll need to troubleshoot directly, see the private message section.