Outlook 2016 email and events not showing in Spotlight preview
I was able to solve a similar problem by updating the timestamp on all my Outlook mail files.  After that Spotlight properly indexed them.

I used a terminal window, went to the Outlook messages folder using the following command:

cd ~/Library/Group\ Containers/UBF8T346G9.Office/Outlook/Outlook\ 15\ Profiles/Main\ Profile/Data/Messages


and then used find command to touch the files.

find . -type f -name \*.olk15Message -exec touch {} \; -print