Interesting I’ve always thought mail.app has had amazingly good search. I recently moved away from it though because I wanted a more flexible workflow and tried a quite a few other clients. The “pretty” macOS mail apps had not very good search functionality so they were unusable for me.

In the end I went with Mailmate and I’m extremely happy with it.

I like to use Mailspring [https://getmailspring.com]. It does what it needs to do, without too much fuss. The UX is clear and I can connect multiple email accounts and use the unified inbox without any issue. I am in no way affiliated to the product.

Try mutt. Classic, simple, scriptable, it's got everything you could ever want!

https://en.wikipedia.org/wiki/Mutt_(email_client)

Depends on your definition of 'desktop', of course ;)

outlook. everything is looks good but littered with bugs and exceptionally weak dev teams behind it.