Well, I’ll admit I haven’t looked into this too much, but I do have a few that I know of.

First is a fork of Thunderbird ironically called Betterbird. Looks like it’s multi-platform. For Linux it’s also available as a Flatpak.

Another desktop alternative that I’ve known of for a while is called Evolution. This one appears to be Linux only, unless I’m mistaken.

There’s also one called geary; also Linux only. The only options I’ve found for it is an unverified Flatpak or building it yourself.

While I haven’t used it personally for Android, I’ve heard good things about K-9 mail. Can be found on F-Droid.