If the search index is not working on Outlook, please try rebuilding the index by following the steps listed below:

1. Client Side
a) From Outlook, click File, Option, search.
b) Select Indexing options, select Advanced.
c) Under Troubleshooting select Rebuild.

2. Server Side
a) On exchange execute the below command:
New-MoveRequest [email protected]
b) Execute below command to verify the status
Get-MoveRequest -identity [email protected] | Get-MoveRequestStatistics

Thanks,
WintelAdmin