Outlook for Mac uses OS X Spotlight to search your mailbox. Occasionally you might run into an issue where you can’t find something in Outlook that you know exists! This might be related to Spotlight indexing issue where it’s incomplete or corrupt. See our KB article that applies to search in Mac Outlook in general: https://support.microsoft.com/en-us/kb/2741535/.
To fix this, follow the instructions in the KB or steps outlined by Apple to re-index: https://support.apple.com/en-us/HT201716
Alternatively, you can:
When Spotlight on OSX doesn't behave as expected, re-indexing the Outlook 2016 for Mac search database can help.
1. Restart Mac, so that it restarts the spotlight services
2. Go to Finder > Applications > Utilities > Terminal.
3. Type "mdimport -L".
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, again restart your Mac, and go back to Step 1.
4. In Terminal, re-index your Outlook database using the following command and substituting your own user name for <user_name>:
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/Main\ Profile
Note in the command above, the path after "-g" is the default path where your Outlook is installed. The path after "-d1" is the default path to your profile. You'll have to substitute your actual paths if you have renamed your profile or installed Outlook in a different location.
5. Re-indexing will take some time to complete. Once the process is complete, quit and re-launch Outlook.
If the issue persists after the above steps, please go to Terminal and run this command: mdimport -L 2> ~/Desktop/spotlightlog1.txt. Get the file spotlightlog1.txt on Desktop and contact us using Help > Contact Support (currently available to Insiders).