Hamilton Chua asked me to post the patch for the spam module.
My patch basically turns off the option that you can insert three different versions of your spam (aol-html, html, plain text) into the spamimg interface, which then get sent to a user according to the guessed email preferences (see TABLE users_preferences - email_type), which are plain/text most of the time and thus the problem...
This is the way to the patch:
https://openacs.org/sdm/one-patch.tcl?patch_id=42