Same issue as all mention above. Deep trouble shooting using terminal mdimport and mdutil commands to no avail. Spotlight will not index OL 2016 mesgs regardless of what steps taken. The only workaround / recourse I have had is to install vmware vfusion, stand up a Win 10 VM solely to run OL so I can search emails.
The net is full of posts concerning the topic with all kinds of attempts to resolve. Here in apple land apples response is nothing but crickets?
2019 Pro Big Sur 11.2.3. 2.4 32G 2667 DDR4
The only fix I have not tried is to backup to Time Machine and reinstall Big Sur and try again.
Questions Apple? Can I get reimbursed for the Win10 Lic or vFusion I had to install so I can perform the basic function of searching in emails?