You definitely _think_ you're sending in HTML, but that might not be the case.
Are any of the recipients in your Contacts folder? If so, open one of the
recipients' Contact record, then double-click the E-mail address. You should
see a dialogue labeled "E-mail Properties". At the bottom should be a
drop-down labeled "Internet format". If that says "Send using Outlook Rich
Text format", that's the problem. Change it to "Let Outlook decide the best
sending format." Do that with each contact receiving the WINMAIL.DAT file.
This can be the recult of importing contacts. Is that what you did?