When I try to search for an email message using Spotlight, I get "No Results". Have tried the various solutions in this forum, none of which work. Specifically, tried to force a reindex by copying the identity into Spotlight Preferences - Privacy tab, so that I could then remove it. Got the following error message: "The item couldn't be added or removed because of an unknown error."

Not sure if this makes a difference but Outlook is tracking more than 2000 folders across 3 email accounts, and tens of thousands of messages. But my main Inbox is under 100 messages.