Search results may be incomplete because items are still being indexed.This issue affects Windows 7, Windows 8.1, and Windows 10. It also affects all versions of Outlook.
Microsoft has fixed this issue with a new update released on June 27th. You can download the update here:
- In Outlook, click the File tab, and click Office Account.
- Click Update Options and click Update Now.
To review Microsoft's documentation on this issue, see Issue #5 on this page: