Rich Text format is proprietary to Outlook.  Most, if not all, other e-mail
programs, including Outlook Express, are unable to process messages sent in
Outlook's Rich Text format.  (I have heard that Eudora may be able to read
RT, but haven't tried it myself.)  If you try to send a Rich Text message
with an attachment to a non-Outlook user, the Rich Text formatting will
usually end up as a "winmail.dat" attachment on the recipient's end, and the
original attachments will be lost.

On the other hand, sending Rich Text messages between Outlook users should
work fine, and is actually required when sending Outlook items to other
Outlook users.