Outlook automatically uses the format of the original message when replyuing
or forwarding.  You can't change that.

I just figured out how to do the opposite, so I might have a solution for you.
Outlook users sending RTF or HTML messages to users with an email reader
that could only read plain text would turn up blank or full of html tags.
This is what I did:

I set the following options:
Tools->Options->Email Options(within the preferences tab)
I checked the 2 boxes: "Read all standard mail in plain text"
                                   "Read all digitally signed mail in plain
text"
Then, under Mail Format tab,
I chose "Compose in this message format":
Internet Format: "Plain text"

then, I clicked on "Internet Format"
a new window came up and under the "Outlook RT Options"
"When sending Outlook rich text messages..."
I chose the option "Convert to plain text"

If you make your settings so they are not in plain text and put them in RTF
or html, it might solve your problem.

By the way, I am using an IMAP connection to the mail server. I believe
these settings can be made on the exchange mail server.
Good luck.
Irene