What Outlook calls rich text is a proprietary Microsoft format, where the text and the style data get bundled up in a non-standard way and sent in a winmail.dat attachment. Mail can do rich text or plain text, but what Mail calls rich text is actually HTML. Both software are pretty much straightforward to use: download, install, and you’ll be able to unpack any winmail.dat file just like any other archive (zip, rar etc). TNEF’s Enough by Josh Jacob if you’re using a Mac OS X 10.4 and above.
The instructions in this FAQ apply to both the old and Digit user interfaces.