I use Thunderbird because I get roughly the same user experience across all platforms. For linux-only I've heard a lot of users love evolution, though I've never daily-driven it myself. I can't even answer whether it meets your requirements, but wouldn't take long to install it and find out for yourself.

I can't speak from personal experience but Emacs has RTL support, is themeable, and there are a few plaintext email clients implemented on top of it that could work for you.

@farooqkz you can check claws-mail. But maybe you can't use the intern editor. Just try it.

I tried Merkuro and KMail but Merkuro didnt do anything and KMail couldnt be setup with the weird IMAP server of an organization

Still worth a try though and Qt for sure has RTL and other scripts

Given all the options, Thunderbird and Evolution seem to be the best. So I'm sticking to Thunderbird...