Just looking at the stats on https://emailclientmarketshare.com, and it's always puzzled me that Apple Mail is around 8%.
From some UA sniffing work we did a year ago (hence 10.10.3) we saw the UA of Outlook 11/15 and Apple Mail as the same:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_3) AppleWebKit/600.5.17 (KHTML, like Gecko)

So do we think that the 8% is really Outlook 11/15 combined with Apple Mail? Just curious as I'm putting a deck together where I'm showing the state of client usage and I'd hate to look bad...

Perhaps a member of the litmus team can shed some light?