One thing you can also try is changing the turns file extension from .txt to something else.( .cm for example). I was having trouble with one of my opponents not getting turns that were over 100k in size. This seems to have resolved the problem.
As an added note, the game does not seem to care what the extension is, so there is no need to change it back to .txt on the receiver's side.
As a couple others noted, the only opponents that I have that it will come in as raw text in the message body are sending from yahoo accounts. They have to zip them, no one else does. And I think I have finally converted them to gmail, that is the final solution!!!!!