PBEM - map files?

[Mobhack]

Of course not. You only need to send the map to the other player if he doesn't have it installed.

And I think, not even then since the map data will be sent wrapped up in the PBEM data file from the originating player. Not something we have tested, but someone who PBEM games using set maps may have some knowledge of, or a simple test with your opponent could determine.