Even embracing its manifold faults, I'm still a fan of Microsoft Exchange server.  It has been a tempestuous relationship of the years but I'm still there. 

However I have recently had cause to look at MailEnable and I have to say I am very impressed. The web mail is better than I expected and I think it's a keeper.  There are still a couple of configuration items I need to get straight but initial impressions are good.