Thunderbird is alright but I think it's in need of what Firefox was to Mozilla Suite – a lean and mean cross platform IMAP client with a nice UI, and that's it. I don't want an Outlook-style client that's full of things that aren't email (calendar etc).

Recently I've had delivery issues with Thunderbird. That makes me rethink my use of it.

I sent the same email from the same address to the same recipient once with Thunderbird on Windows, once with Apple Mail on iOS. The one with Thunderbird was silently dropped (not even reaching the spam folder), the one sent with Apple Mail reached the inbox without issue. I've had that same issue with 2 different recipient companies.

And yes, my address passes SPF, DKIM, has a strict DMARC, and isn't in any blacklist (uses iCloud+ as host).

Also Claws Mail for Windows:

https://www.claws-mail.org/win32/

(The URL is /win32/, but there are 64-bit builds there too)

Installed it again today. I use Windows only for gaming and therefore didn't care much about mail on it.