The code snippet needs to be edited. The last line of code removes the JRoute::_ call around the index.php
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('index.php'); /* Updated line of code */
I just tested this on my test site and it worked for whether the domain matched or not. However, let us know if you run into any problems with the change.
I've made an issue in the tracker to add an option for a future release.
-Melissa