The replacement Surface 3 I received from Microsoft had Windows 8, rather than Windows 10 pre-installed. I received the unit on Friday and on the first boot I saw a message box saying the device was eligible for a free upgrade, but I had to wait until Sunday morning before it downloaded Windows 10 for me to install.

I upgraded the replacement Surface 3 to Windows 10 today, and right now I am trying to confirm the installation and I am having problems getting the Mail app to load. The Surface is acting like it is not successfully logging in to my Microsoft account.

I am pretty certain the issue has to do with logging into my Microsoft account. After another restart, I successfully logged in and was able to get the Mail up to run. A subsequent login after the fact was very slow, so either it took a long time to authenticate, or at some point Windows 10 gave up and used cache credentials.