The best FOSS mail apps I came across, are

1.) Evolution (Gnome / Linux)

Although it may not be the shiniest star, it has a huge feature set and works pretty good for me.

2.) Mailspring

It is a pretty good Mail.app Alternative, but I did not like the restrictions and the user interface was pretty unpolished, although pretty Mail.app-like

The most annoying part on all Mail apps I tested was the search. It was mostly unusable, if you have ever used macOS Mail. The search is so much ahead of every other mail app I tested (and I tested a ton) that I don't know if it is just hard to build it or no one ever tried to rebuild the Mail.app search. The only app that was pretty close was mutt/neomutt, but although I like terminal interfaces, in Mail I often use Mime features like bullet lists, tables, Image previews and other stuff that is hard to do in Terminal UIs.

If like the search features of Mail.app, MailMate is even better: https://freron.com/

never heard of it, but I think being satisfied with macOS Mail does not need an alternative for macOS, but for Linux ;)

But I'll take a look.

I'm fully satisfied by Claws Mail, however thread oriented message grouping like Geary would be a nice improvement, along with a sleeker UI. I would not however give up too much performance for it; Claws Mail speed to me is one of its major selling points.