When sending a message in MAPI, if I want to force a message to be plain
text, I delete PR_HTML, PR_RTF_COMPRESSED, and PR_InetMailOverrideFormat,
and make sure to set PR_MSG_EDITOR_FORMAT to plaintext. You could probably
do the same for incoming messages. If you do all that, also make sure
PR_BODY is set so the user can see something.