Looks ike Microsoft Mail and Calendar team heard the voice of the users complaining about Mail app sync problem in the latest update. Last update to the app caused syncing problems to many users.
So, Microsoft is pushing out a new fresh update to insiders in the fast ring with version number bumping to 17.6965.40522.0. The update tends to solve the issue faced by many insiders.
Head to the store to check the fresh update!