I didn’t find one! Currently I use Thunderbird with BiDi Mail extension, but it’s not good. This extension is not updated for latest Thunderbird 3 Beta, and it is buggy there. But anyway, I still use it as other alternatives are not better too.
I wonder why none of these mail clients support that! Thunderbird should support it by default. At least, mail clients should have buttons for setting the paragraph direction. This is not a big request! Adding 2 buttons for setting paragraph direction in HTML messages is not too much IMHO!
Evolution: it doesn’t provide the buttons too, but it tries to automatically detect paragraph directions and put correct HTML tags for it. Well, till now, it’s good. But it has another problem: I tell it to use UTF-8 encoding, but it doesn’t care! It encodes Persian characters using HTML unicode characters, which is not desirable as it increases the size of email considerably. I’ve filled a bug against it long ago, but there is no activity to solve it. Interesting!
KMail: Do not provide any kind of support for RTL languages AFAIK.
Well, it seems that I’ve not tried anything else! Do you know a good email client which supports RTL languages correctly?!