I have the exact same issue. Got a new macbook air, with Big Sur last week, from work. Since then, only problems with mail app. Search function worked the first day. Next time I logged on, didn't work whatsoever. And the mails from post@ and admin@ were all mixed together. I managed to fix both, but then next time I logged on, same f problem. Happened three times now! The mails today were all in both post@ and admin@ boxes... (all together) ...suddenly that fixed itself after an hour... But search function is gone again. And I use it a lot at work. I had fixed it with a "recipy" found on here, but yeah only worked till the next time I opened my laptop... No restart or anything prior to that... (Restarted during the recipy ofc. But not again after it worked) I see now the next update for Big Sur is on tonight! Im scared of getting even more trouble! The update does not say it will fix any bugs in mail app.... What to do?? Same problem as Sharfin83 and swmartinez it seems. One more thing... Quicklook also have issues... works some places in finder and mail etc, but not other places in finder. Ive had issues for a week now with stuff and not been able to work (work from home in a small online..company)