Not sure if this is a PHP issue but here is my problem:

I have a PHP application, coded by myself and distributed for use. A user installed this on a VPS server. In my application, there is a contact form that uses mail() function to send the contact message to the specified email address of that user. That email address is set in a settings file. When I test this contact form with my email (...@yahoo.com), another email (...@live.com) and one other that uses my domain (...@mydomain.com) it works fine, the contact message is sent. However, when the user puts his email which is (...@hisdomain.com), the form doesn't send the message. No errors. What could be the reason for such a behavior? I am guessing that it is something to do with his email but no idea.

Thanks for any ideas.