> There is also Open-Xchange
I think most people can safely rule-out Open-Xchange unless they've got deep pockets.
Sadly commercialism has taken over there, and their sticky hands have now reached Dovecot.
The open-source version of Dovecot 3 will have the vast majority of multi-server functionality stripped from it. OX have stated on record[1] that their plan is that open-source Dovecot 3 will supported and maintained for single-server use only. If you want multi-server you will need to pay $$$$$$$$$ for the commercial version.