The Mail app is a great standard app in Windows 10. Although not as rich as the Web portal, it’s a very handy offline client to have when you’re not connected to the internet.
I recently discovered my email wasn’t synced. I didn’t see any old email either. Only the message “We didn’t find anything to show here”.
I did remember that I had modified my Privacy settings a while ago, so I reverted those changes to their defaults: Settings -> Privacy -> Email -> Choose apps that can access and send email: On (2x)
However, my Mail app still didn’t sync. The caveat in this issue was the Calendar setting:
Settings -> Privacy -> Calendar-> Choose apps that can access calendar: On (3x)
After allowing apps to access my calendar, Outlook mail was able to sync.