You can use a hex editor on your Combat Mission Executables and change the file extension that CM puts on the file.
I have done this for All three versions and it works like a charm.
The game does not care what the file extension is when opening your turns for play.
See the instructions attached, feel free to email me if you try it and have any questions.
-Note: You can also simply do this manually by renaming the turn - remove the .txt , replace with something else. I choose the hex edit so I would not have to do this with every turn.
In Das Tiger's case it is most likely your opponents Mail provider that is causing the embedded text.