I didn't notice it either until I was replying to an email that had all kinds of formatting in it---that formatting was shown in the mail window, but as far as I can tell there is no way to change it. I also had to cut and paste some text from another email into a new message, and the pasted text was in a different font and different size. Again, there was no way to change it, so I had to send the message off with wonky fonts. So although the text you input does seem to be plain text, the app will actually use more than that. Weird...