For anyone searching for how to change this you can comment out the if statement in the following:
include/class.mailfetch.php
//Allow mismatched emails?? For now - YES!
//if(!$ticket || strcasecmp($ticket->getEmail(),$var))
// $ticket=null;
It happens often enough for us that different customers at the same company work on the same ticket that we wanted to ignore the email address it comes from.