Update: Finally got to build 10074 after resetting Updates. Mail is working fine now.
I updated the machine I have Win 10 on today, flawless upgrade and works GREAT.
Finally got this one installed, had a few problems before but got there in the end. Mind you I don't turn it on that much, I'll start out in my normal 8.1 and get quite involved in my emails and forums that by the time I think about 10 it's really to late to start it up.
I'm typing from it now but there's no saying I'll use it again tomorrow.
I don't bother with the built in Mail, I've got Windows Live Mail installed as I like the way it handles all my email accounts and RSS Feeds.