I have been using ModusMail by Vircom (for Windows) for a number of years with great success. There are two primary reasons that I use it. One is that it is an extremely feature-rich, mature product. It can integrate with just about anything else that I need, supports secure connections, and has a very, very good anti-spam solution that is constantly being updated by a dedicated team of professionals, etc. The second reason is that it requires very little maintenance. The software pretty much runs itself. We give customers access to manage their own mailboxes, we generate reports for accounting so they can bill accordingly, and hardly ever have to spend time "managing" the solution. For larger enterprises or situations where e-mail absolutely cannot go down, Vircom also offers a cluster solutions where two of them can work in tandem to ensure maximum uptime.
Now for the cons. No system is perfect, so there are bound to be some things that people won't like about any solution. For ModusMail, the first is cost. It is expensive, yes, but worth every penny in the amount of administration time we save by using it. The second is that is it proprietary software. For me this isn't much of a problem, but some people would scream bloody murder if they can't see the source code, so that is a potential issue. The other con is that is is a pure e-mail server, not a collaboration platform like Exchange. ModusMail handles e-mail via SMTP, POP, and IMAP. It ships with a webmail application which includes basic contacts, but that's about it. If you're looking for group calendaring or other collaboration functions, ModusMail is not for you.
As of this writing, ModusMail is at version 4.6, so some of the above may not apply if you're reading this after a new version is released.