email and calendar app for both MAC and PC

I have purchased a HP MediaSmart Server for my small home-based office. There will be 4 computers which access the server-2 macs (all running OS 10.5) and 2 PCs (1 running Vista and 1 running XP). Does anyone know of an email and calendar app which can be accessed by all computers? The server runs on MS Windows Home Server.
Thanks for any suggestions.

Eudora
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You can download the Beta8 linking from there
have a look at this page also Eudora Sharing Protocol

Mulberry is a good one
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