Opinions needed to choose between a Mac OSX Server or Windows Server for business.

[fangpyre]

Give Mail a try.
I admit its not as strong as Outlook.
But its much better than you think.

And more important, it is lighter (i.e. faster), more stable, and the fact that it doesn't store your emails in 1 big file is a huge stability concern.
Here, you stand to lose 1 (or a few) emails rather than everything.

It also has a few features that I like.
Such as inline video and audio, and highlighting conversations.

[hakka]

Mail app is like Outlook Express in Windows. Simple application just enough to get your mails.

For OSX there are several clients, Microsoft Entourage and Lotus Notes are the most developed for business IMHO.

[fangpyre]

Microsoft Entourage is also very basic, and offers little function over Mail.
Lotus Notes on the Mac, I've never used.
But Thunderbird is not bad.