So, I want to offer Exchange-like email services - specifically, I want to be able to handle server-side calendering for Outlook clients... but I want to do this without running Microsoft Exchange. I'm currently using Exim, but I'm not locked into this by any means. I could use sendmail, qmail, postfix... whatever.
Anyone have any suggestions/recomendations for accomplishing this task? The solution doesn't have to be GNU.. I'm okay with paying license costs for a commercial solution. I just want to have some confidence about which one I choose.