I usually send my essays to my Uni tutor by e-mail the night before my tutorial. With Entourage I never had a problem. However, I now use 'new improved' Mail. I sent a Word document to my new tutor, and she couldn't easily open it - I had to rename the file with .doc on the end for her to make Word open it, and then it came up with loads of code, with my document buried in the middle of it.
I think she was using quite an old version of Word, but .doc is still .doc, right? I was wondering if Mail used compression of some sort which was causing problems?