Sadly and somewhat ironically, Apple's own mail server software does not necessarily provide the best experience for Apple clients (it goes without saying this is even more true for Windows i.e. Outlook clients). Apple are getting better and OS X Server combined with Profile Manager is now very close but is limited to just Apple clients.
Historically, Kerio Connect has been a popular alternative choice to run on a Mac server instead of Apple's own software. Kerio has the advantage that it can act as a full-blown ActiveSync compatible server for iPhones etc. and also works as a fullblown Exchange style server for Entourage or Outlook 2011 (for Mac). Frustratingly Kerio have crippled their EWS (Exchange Web Services) implemenation so that while it works in Outlook 2011 (for Mac), it does not work in Apple Mail which also supports EWS. You have to use Exchange-IMAP instead for Apple Mail. It also requires installing a 'connector' plugin on Windows machines as it does not support MAPI.
It is my opinion that the best user experience for Mac, iOS, and Windows is currently a 'real' MS Exchange server but of course that is a horrendously expensive option. By best I mean full featured, no need for installing additional plugins and from a client point of view simple account setup, on all three platforms (Mac, iOS, and Windows) one can simultaneously setup Mail, Calendar and Contacts. Yes one can do this with Kerio but doing so requires using their setup tool and/or Outlook plugin.
As Kerio's support policy has also become less friendly, I am interested in hearing of alternatives even those that can only run on Linux or a VM. So far it seems that while some support ActiveSync, and some support MAPI, none seem to support ActiveSync (needed for iOS), and MAPI (needed for full Outlook compatibility), and EWS (needed for Apple Mail and Entourage/Outlook2011).
This would seem to rule out contenders like OpenChange, Zimbra, Kerio, and Communigate Pro.
Does anyone know of a better solution?
PS. Supposedly Exchange 2013 will kill off MAPI leaving the choices of EAS (Exchange ActiveSync) and EWS (Exchange Web Services). So a solution that supports just those two options would be fine looking to the future.
PPS. Just come across SmarterMail. Like nearly all these products it is very hard from their websites to definitively determine whether it supports EWS and/or MAPI but it seems SmartMail does support EWS and ActiveSync. Anyone any experience of it? This link is also interesting http://www.parallels.com/uk/products/plesk/smartermail/