A HUGE second for SOGo
We used it for many years in a Gentoo/Dovecot/Postfix environment.
It was super fast/snappy, and extremely reliable, and works perfectly with both Thunderbird AND Outlook (this was a huge plus for some of our users who ridiculously preferred Outlook)...
they also offer implementation and ongoing support services at very reasonable rates, but if you prefer to do everything yourself, the documentation is perfectly adequate, and their email support list should address any potential issues you might have.
SOGo rocks…