Mail sent with the MUA Mutt 1.5.21 will not display correcly with Android 2..* stock "Mail" application. The mail appears to be empty, while the actual mail content is found in an attachment, sometimes called 'Unknown.txt'.
This happens because of the "Content-Disposition" line added by Mutt in the header of the mail. I patched Mutt to remove this line on my machine, and then mails would display nicely on the "Mail" app.
However this is not Mutt's fault. The Content-Disposition is perfectly legal. (And it works with almost every MUA out there). So I guess Android "Mail" app should be fixed to handle this line properly.
Note: I do not have any Android devices nor am I an Android developper, so forgive me for the little details regarding Android.