I'm also using Mail.app on my macs and while I never had those problems I do see people complaining about those things a lot and I'm curious what would cause that. Apple's radar/bugreport/feedback stuff is hidden so that's somewhat sad (but understandable) but maybe it turns out most people have the same problem due to similar context (account setup, data in use or something...).

The most heavily loaded Mail.app I seem to have is one with two MS Exchange accounts, iCloud, a couple of IMAP accounts and a single POP account. I generally archive everything older than ~10 months out of my inbox leaving a combined 43k messages in that virtual inbox 'group'. Maybe people with larger message stores trigger some programming fault?

I have a love/hate relationship with Mail. It has exactly the feature set and UI I would want from a mail application.

But god, these bugs. Somwhere down the line it became strangely slow. It's constantly displaying false values on how many emails it is about to fetch. And I accidentally deleted emails on more than one occasion without having a clue on how that happened.

I'm also using Mail.app right now and I've been having similar issues with search and stuff. Does anyone have any recommendations for good desktop email apps on macOS?

I remain convinced that somewhere inside of Mail.app someone decided to try to speed up search with some approximate index or badly rewritten strcasecmp.

Mail not being able to properly find an exact string match correctly means I had to give up and just use GMail in Chrome and on iOS. Once GMail on iOS added IMAP and SMTP, I never needed to go back.

I have used Mail.app and the default mail app on windows 10 a lot. Can you believe that the default windows mail app is even worse? Very basic features are still missing after many years...