I don't think you'll get the winmail.dat stuff with this message, I'm an Outlook user also, but I think the answer is thus:

REMOVE the check mark in your Address Book (provided your addressee is "in" your address book) next to the phrase that says:

"Always send messages in Microsoft Exchange rich text format".

I used to have this problem before, but after I removed those check marks from all addressees in my address book, the problem went away.

If this message generates a winmail.dat for when you receive it then something has changed in Outlook/Exchange.