Do we know if the Android Gmail application sends the same text as both plain and HTML as many clients do or does it just send HTML?
Honestly, Not a clue, can send you an email from my android, if you'd like to investigate
One idea would be to have incoming mail processed so any HTML part is dropped and the plain text version used instead. I am sure I have seen some other mailing list do this. Interestingly some SPAM senders use mail with text and HTML parts where the mime headers indicate the two parts are alternatives whereas in fact the text is reasonable text designed not to alert a filter and the HTML is the real advert.
This would be a great idea :-) Google would certainly receive a lot less spam on their groups from me (about the android not sending non-html emails) hehe