Outlook 2003 Windows

I sometimes mail to recipients want to only receive plain text messages.
How to I set this in Outlook?

That is, if the recipient is , Outlook
would detect an attempt to send HTML or RTF format and either change it to
plain text before sending or prevent me from sending the message until I
change it to plain text.

I looked at the rules wizard, but that only offers me an opportunity to
process an outgoing e-mail after I've sent it.

Hope this helps,
Charles Belov
SFMTA Webmaster
http://www.sfmta.com/webmaster