If you are on macOS Mojave (10.14), you can follow the steps below to reindex Spotlight for Outlook for Mac.
~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/
- Choose Apple menu, then System Preferences, and then Spotlight.
- Select the Privacy tab.
- In Finder:
- On the Go menu, select Go to Folder...
- Copy and paste the following location into the "Go to the folder:" dialog box and select Go
- Drag the "Main Profile" folder to the Privacy tab. Once added, remove the folder, and Spotlight will re-index the folder.
You must perform this step for any additional profile folders you have.