David,
I really apologize for the delay. We usually don't let things linger so long, but this issue just took a little time to make sure the code we recommend is the right code. We've done it before, and frankly, it probably should be an option in our settings.. but that's a discussion for another day.
Anyways, here's the code that should do what you're looking for. Please edit the /components/com_jfbconnect/models/loginregister.php. Around line 219, you should see:
$newEmail = $profile->get('email');
$fullname = $profile->get('full_name');
Right after that, add the following:
if (strpos($newEmail, '@domainname.com') === false)
JFactory::getApplication()->redirect(JRoute::_('index.php') );
That's it! That first line simply checks if the email contains the proper domain name (update it as necessary). If that domain is not found in the email address, the next line will redirect the user back to the home page. You can update the index.php to a full non-SEF URL (index.php?option=com...) if you want to redirect elsewhere.
Please test and again let us know how it goes.
Thanks,
Alex