Does anyone know of a method whereby we can reject messages that are
being relayed that are supposedly coming from the recipient? Eg: sender
= user@domain.com; recipient = user@domain.com. Any such restriction
would come after permit_mynetworks, since of course we don't mind if
people send to themselves internally.

I was thinking of header checks, but since they're evaluated
line-by-line, perhaps it's not possible. And if we can, I wouldn't have
the faintest clue where to start. Does anyone have any ideas?