Bad & good news.
The bad news is such functionality is not configurable or supported yet. The good news is, it takes only 2 minor code changes to get it working as you wish.
In open.php line 24
change
if(($ticket=Ticket:($_POST,$errors,SOURCE))){
to
if(($ticket=Ticket:($_POST,$errors,SOURCE,false))){
In include/class.mailfetch.php line 237.
change
if(!($ticket=Ticket:($var,$errors,'Email')) || $errors)
to
if(!($ticket=Ticket:($var,$errors,'Email',false)) || $errors)
How sweet is that?