"Rebuild the Index
If nothing else works, you may need to rebuild your index. This is easy to do, it just may take some time. All you need to do is follow these steps to rebuild your Outlook 2016 search index:
Click on File, then Options. Click Search on the left hand side
Now click on the button that says Indexing Options
Click the button that says Advanced
Within the tab that says “Index Settings” click on the button that says “Rebuild”
Note: When the index is being rebuilt, your search will not work. You should wait until the index is done rebuilding. The time it will take for the index to be rebuilt depends on the system you are using, the amount of files that need to be indexed, and file size.
When your index has finished rebuilding, you can go ahead and restart your Outlook searches. This should have fixed your Outlook 2016 search problems."