When I posted about the Zimbra collaboration suite a little while ago, it was noted in the comments that, while the Zimbra client is now working in Safari 3, the server isn't yet Leopard-compatible. Kerio MailServer offers a Safari 3 and Leopard-compatible alternative with a lot of groupware functionality.
Kerio's Safari 3 compatibility stems mostly from the fact that it doesn't have a rich text editor in the client, but it works flawlessly for plain text email and the interface is as smooth and AJAXy as Zimbra's. Kerio has had a Mac version of the server since 2003, with drop-in Exchange replacement capability and the ability to sync contacts, calendars and email without the need for an external client. Kerio has even had iPhone sync since last Fall.
Kerio's pricing structure is slightly different from Zimbra's, but competitive; there's no open source Kerio version, as there is with Zimbra. But if you're in the market for a Leopard-based mail server and collaboration solution, Kerio seems to be a pretty good choice.