Check Microsoft Outlook in the indexing options and save the changes. Now you need to enable Outlook Indexing in Search Settings, which was previously disabled. Open an elevated PowerShell console and restart the WSearch service:

Check the box Allow this file to have context indexed in addition to file properties -> click OK.

Right-click the OST or PST file you need and select its properties.

Select files one by one and click Open file location.

Open File -> Account Settings -> Account Settings.

In the properties of Outlook OST and PST files, make sure that their indexing is allowed (it is worth doing regardless of their indexing being already enabled in Windows Search Service settings).

Close Outlook, and using the Process Manager, make sure there is no outlook.exe process running on your computer.

Disable the indexing for Microsoft Outlook.

To solve the problem, you should temporarily exclude Outlook from items that are scanned by the Windows Indexing Service.

Enable Outlook Indexing by Windows Search Service

If you haven’t made significant configuration changes in Outlook for a long time and indexing is still ongoing, then the problem with the Outlook index is obvious. If you have recently (up to 3-4 hours) performed these operations, you need to wait until indexing is complete. The Outlook search index is rebuilt when you add a new mailbox or Outlook Personal Folder (PST) file, move emails between folders, etc. Outlook is currently indexing your items.

Ms outlook 2021