Bernard Peek wrote:
[snip]

Outlook is a MAPI client which interfaces directly with Exchange Server.
The MAPI standards are part proprietary and wholly opaque. But Exchange
is still the default mail handling system even for many companies that
could implement a UNIX based mail system if they wanted to. The stumbling
block is that Exchange/Outlook is a very effective calendar system, for
instance enabling users to compare many people's diaries to find a time
when all of them are free.

As far as I am aware there are no UNIX based equivalents.

Apple's Calendar and Mail applications work with Exchange and are
interoperable with Outlook - at least until M$ change the game again. OSX
is built on BSD.