If you have a partition with OS X 10.4 on it, why not use the included Mail program? It's interface is very similar to Outlook's.
The problem is that many of the e-mail programs for OS 9 are no longer available...discontinued about 8 years ago.
If you must have something for OS 9, see if you can find an OS 9 version of:
Eudora (free, but no longer being developed)
http://www.eudora.com/email/index.html
__________________
Randy B. Singer
Co-Author of The Macintosh Bible
(4th, 5th and 6th editions)
OS X Routine Maintenance and Generic Troubleshooting
http://www.macattorney.com/ts.html