Why is this happening?

[Bear]

What you display is what you get if you open the pbem file with notepad. If he zips the file without messing with it, and you extract that file into your folder, it should work. I other words, don't open the file.

The file is coming in a .zip (winzip), the best way to send turns, good.
But I suspect that when your opponent is ready to insert his turn into the zip he is attaching all system and hidden from his rig to yours...that's a goof up.

On the Winzip ADD window, that's waiting for a file to be assigned, there is a check on the lower part of the Winzip window that needs to be unchecked (reads: "Include System Files and Hidden Files")

The turn file is all contained in the zip, safe from corruption, and is from your opponent.
If the turn file is corrupt, not in notepad, or just wrong THEN it's your opponent that is sending you something wrong.

Ps. Always make sure you're both using the same game version.