I'm glad you asked.
From Tools, choose Address book.
Show names from your Personal Address book if this is not the default.
From the list, choose your entry for this list.
From File, choose Properties.
In the properties dialog, choose the SMTP - Address tab / page.
Below the Alias and SMTP fields should be a checkbox in front of the
label "Always send messages in Microsoft Exchange rich text format, and
since you are sending rtf, this is probably checked. Uncheck it, and
choose OK.

>----------
>From:  Keven Miller[SMTP:[log in to unmask]]
>Sent:  Friday, October 03, 1997 10:09 AM
>To:    [log in to unmask]
>Subject:       [HP3000-L] MS OutLook and WINMAIL.DAT
>
>I was informed that Im sending the uuencoding WINMAIL.DAT
>and the end of my postings. I am using NT4.0 MS Outlook thru
>Exchange. Anybody know how to disable this?
>
>********************
>* Keven Miller   KC7LYD
>* [log in to unmask]   (Salt Lake City, Utah, USA)
>********************
>