A not small point: Microsoft 'supports' IMAP for appropriate values of 'support', but they break one of the two major feature of IMAP. There are two big features of IMAP that make it superior to POP3:
1) The ability to read, write, reply, delete, and file mail on one computer and go to another and have exactly the same view of your mail. Microsoft Outlook supports this feature well.
2) The ability to do the above on *local* copies of your mail and mail folders and then 'sync' these up when online service is available again. This MS does NOT support. You can only move mail between folder and such when acting on the server hosted folders. (Yes, I know, technically this is an optional feature and an implementation is not required to support it. OTOH, I know of no other MUA that 'supports' IMAP and does not support offline use.)
There is exactly one reason that MS does this and to sound like an echo of Carroll, it is not because MS is incompetent or lack the resources to implement the feature; they do it to sell Exchange servers. And it works to do that. It can even get enthusiastic M$ bashers to deploy it.
I serve a user population that does a lot of traveling, a lot of it between Houston and Tokyo. Those people like to work on their email in the air and prior to deploying Exchange (which I was loath to do), they all used POP because they could work offline with it.
(And please don't mention all the pathetic Open Source Exchange 'replacements'. They all suck eggs and Exchange just works(tm) )