Does it redirect to another page after sending the email?
Add the following code to the page it goes to after sending the email:
<?php @session_start(); var_dump($_SESSION); ?>
This will write the session contents to the page, one of the session variables will show a status message for sending to each address. If this shows Success for all 5 addresses, then it is an issue with the SMTP Server not relaying the email for the address that is not receiving it.