That’s going to be killed off in favor of Outlook in the near future, from what I understand.
If OP is willing to do a bit of extra legwork and somewhat masochistic, then pretty much any Linux-based mail client is fair game with WSL2. The only one I’ve used lately other than Thunderbird is Evolution, but that was just to test a particular distro’s default offering.
And that's why I'm explicitly noting that they're not FOSS, doofus. Besides, if you're using Windows anyway, using its built-in email client is not a huge stretch.
I mean, yes.
But why would I want my emails also to go through the spyware OS.
What you're saying sounds like "you're already using a OS that tracks everything, giving them your emails at this point wouldn't hurt."
They could already have access to your emails, because… you’re running their OS. They can slip in any code they want and run it with NT AUTHORITY\SYSTEM-level privileges (comparable to root-level privileges on Linux systems).
If you run any other OS you’ll also have to trivially trust the makers of that OS with root-level privileges (or comparable).
(Personally I don’t believe that MS is scanning all your local emails, but they certainly have the technical possibilities to do so very trivially.)
Given people are regularly promoting proprietary trash and being upvoted for it, while people taking a pro-FOSS stance are downvoted, I don't think this is a FOSS community in anything but name anymore.
At the very least I'd have hoped we'd left the childish name calling behind at reddit, but it seems you can't really take the reddit out of the redditor.