I have noticed that if you have iCloud installed and you use voice to ask Cortana to open Mail / Calendar etc, Cortana will open the iCloud app rather than the Win 10 app even when the Win 10 app is set as the default.
When you uninstall iCloud, things return to normal.
Even if you leave the “Mail,Contacts, Calendar and Tasks” box unticked, the problem persists.
Recently I installed iCloud on my Surface Pro 4 (SP4). I really do not have any use for iCloud other than as a back up for my iPhone (excluding pictures). A back up on iCloud makes it very easy to get all of your stuff onto a new iPhone. I don’t use it for backing up pictures as you don’t get much free space and I don’t want to pay for iCloud storage when I have more than enough on One Drive courtesy of my Office 365 subscription.
However, every time there is an update to iTunes, you get prompted to install iCloud. Thinking that it could not do any harm, I weakened and installed it.
I got the answer to the ‘could not do any harm’ thought when I asked Cortana to open my (default Win 10) Mail App, but instead, Cortana tried to open the iCloud Mail App. Naturally, my first inclination was to blame Cortana and/or a recent update. However I asked Cortana to open other programs and she worked well apart from things like Calendar and Contacts which also just happen to be part of the iCloud App.
So I uninstalled iCloud and Cortana went back to opening the Win 10 apps just as I wanted.
Before uninstalling, I checked the list of default apps and this did not show any change from the Win 10 defaults. Also Cortana starts by indicating that these apps will be the ones to be opened but somehow the process changes and it attempts to go to the iCloud app even though I have never used any of these programs.
When you install iCloud, there is an option to sync Mail, Contacts,Calendar and Tasks, but even if this is left unticked, the problem persists. To my untrained eye, it is difficult to attribute this to any attempt to get you to buy more space on iCloud and in fact it is hard to see any benefit to Apple so it more than likely is not deliberate. In my case Cortana voice command is far more important than iCloud – SO — so long iCloud!