Rudi-
Treo Mail is compatible with Mac OS X. However, I'd recommend you research a few mail clients before opting for Treo Mail. With Treo Mail, you can only access 1 account. IMHO, the best email client for the Treo is SnapperMail. It's got a number of Treo specific features, supports multiple POP3 accounts and your not locked into a yearly service charge - you simply pay for the application. Plus, they have a Yahoo support group which can be helpful if you have problems.
You can find a trial version at
SnapperMail.
Good luck.
Chris