Try these steps to resolve mail sync issues:
Ensure Windows 10 is up to date (Start > Settings > Update & security > Check for updates).
Click the Sync button in the Mail app, at the top of your message list, to force the app to sync.
Customize your sync settings in the Mail app (Settings > Manage Accounts > select the desired account > Change mailbox sync settings).
Article ID: 4027851 - Last Review: 29.7.2017 - Revision: 1