This happens on stock modest too;

reproducing:
1. Install theme-customizer and ttf-dejavu, enable this font
2. Create a new mail,
3. In the body write:
a a a a a a a a a a
and so on

Finally you'll find that one letter is cut off.

Sorry for delay, I'm simply a busy student nowadays

One feature request: could there be an option to place a sent message in an IMAP folder?