Upgraded to Windows 10 on a secondary laptop and I'm glad I didn't put it on my primary.
Upgrade went smoothly although the computer is very slow to respond to clicks.
Opening up Windows Mail I see two issues. I don't see an option to 'leave a copy of messages on server' and my contacts are gone.
I previously used Windows Mail (Windows 7 Pro) for email with my POP email accounts.
Now my contacts don't show up.

There's no button to go to contacts to select them and typing contact names in the 'To:' box doesn't bring anything up.