If you search on Outlook and you don’t get results or you get partial results, most probably the Spotlight index is corrupted.

How to fix it:

1. Restart the Mac, so that it restarts the Spotlight services.
2. Navigate to Finder > Applications > Utilities > Terminal.
3. Type mdimport -L.
4. Important: If you see more than one instance of “Microsoft Outlook Spotlight Importer.mdimporter,” delete the Outlook application that you are not using, empty it from the Trash, restart your Mac, and go back to step 1.
5. In the Terminal, reindex your Outlook database by using the following command and substituting your own user name for the <user_name> placeholder:

   mdimport -g “/Applications/Microsoft Outlook.app/Contents/Library/Spotlight/Microsoft Outlook Spotlight Importer.mdimporter” -d1 “/Users/<user_name>/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/<my_profile_name>”

   Note In this command, the path after “-g” is the default path of the Outlook installation. The path after “-d1” is the default path of your profile, where <my_profile_name> is, by default “Main Profile.” You’ll have to substitute your actual paths if you have renamed your profile or installed Outlook in a different location.

6. Reindexing will take some time to complete. After the process is complete, exit and then restart Outlook.

Have a look to the source of this information for further guidance: https://support.microsoft.com/en-us/help/2741535/outlook-for-mac-search-returns-no-results-and-task-items-are-not-displ