[tacticalogic]

OS X Server does have enterprise-capable mail and calendaring functions integrated with the directory, and they're based on open standards so, unlike with Exchange, you're not locked-in. It's included for unlimited clients, as opposed to Microsoft, which charges tens of thousands of dollars to get unlimited clients from Exchange -- per server, of course.

What do they call this application? I'm guessing they're not keeping the users's mailbox in an LDAP directory.

[antiRepublicrat]

What do they call this application?

iCal Server and Mail Services. There's also Wiki Server (collaboration) and iChat Server (a presenter's wet dream in addition to managing IMs and chats). All included, no client licenses.