Ran across this setting while trying to fix my own problems and I remembered seeing your question. No idea if you've checked into it already (or if it'll even work) but have you looked at the ZERO_IS_EMPTY setting in formmail? Here's the help page:
http://www.tectite.com/fmdoc/zero_is_empty.php
According to the documentation, it shouldn't be what's causing your problem, but I figured it's something you could at least try?
Bookmarks