Create a route action to reject incoming emails (ie do not even process them)
Currently, the stop() action merely stops all subsequent routes. The email that matches the conditions is still processed (and therefore charged). As such, a potential attacker (say a determined competitor) can inflict a huge bill on a mailgun domain by sending a large amount of emails to that domain.
My suggestion is to have an action that will ignore a sender / sending domain entirely and refuse to accept some emails with the action.