As others have noted, davmail rocks. It's easy to set up and it lets you use whatever email client you like.
Another important piece of the puzzle for me has been getting the terrible Lync/Office Communicator working in Emacs. After a fair bit of fiddling, I managed to get it working nicely. The trick was to use the build of bitlbee that allows you to use pidgin plugins and let it use the pidgin-sipe plugin which in turn gives you access to Lync. The actual fiddling was to pass a parameter to the bitlbee deamon to sort out an SSL certificate issue and figure out (read: guess) the right parameters to give to bitlbee to correctly add my Lync account.
Once having set up bitlbee correctly with Lync support, I could use Emacs/ERC to chat with my colleagues on Lync. Any other IRC client would do too, of course.