1. Quit Outlook application completely from the Dock by navigating to Outlook on the top menu bar --> click on Quit Outlook.
2. Once done, click on Spotlight icon (Magnifying lens) available on the top right corner or press Command + space bar keys and type 'Terminal'
3. Open Terminal Application
4. Copy and paste the below command in the Terminal window sudo mdutil -E /
5. You will be prompted to enter your Mac password, please enter it and wait till you get prompt/output as Indexing enabled. Then re-index the Outlook profiles folder again.
Quit Outlook completely by click on Outlook Menu > Quit Outlook and then try the steps
1. Choose Apple menu > System Preferences, then click Spotlight.
2. Click the Privacy tab
3. In Finder: a. On the Go menu click Go to Folder...
b. Copy and paste the following location into the "Go to the folder:" dialog box and click Go: ~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/
4. Drag the ‘Main Profile’ folder to the Privacy tab. Once added, remove the folder, and Spotlight will re-index the folder. Note: You must perform this step for any additional profile folders you may have.
See the following article from Apple to learn more: https://support.apple.com/en-us/HT201716
Note: Re-indexing of Outlook profile might take some time (an hour or two) depending upon the size of your mailbox. I'd request you to wait for some time and try searching for an email and verify the outcome.