There are cases where Outlook with Exchange Activesync synchronizes items partially. This article provides information on the root cause of this issue and how to synchronize all the emails in the Exchange Activesync account.
This issue occurs when the Cached Exchange mode the Mail to keep offline setting is configured to a value other than All.
- Open Outlook.
- Go to the Info tab.
- Click Account Settings.
- From the drop-down menu, select Account Settings.
- Under the Email tab, select your account and click Change.
- Change Mail to keep offline to All.
- Click Next.
- In the Test Account Settings pop-up window, click Close.
- Click Finish to close the Change Account window.
- Restart Outlook.
All items are synchronized with the Outlook application.
For more information, see this Microsoft article Only a subset of your Exchange mailbox items are synchronized in Outlook.