Curiously, macOS Spotlight will show email search results but not Mac Outlook. Running Mac Outlook 2016 (version 16.16.12) and 'No Results' is shown when trying to search for emails.
Upgraded from Macbook Pro (late 2016) running High Sierra to Macbook Pro (2019) running Mojave 10.14.5 using Migration Assistant and Target Disk Mode (on old Mac).