There is no solution for the writing of plain text email messages in right to left languages. Thunderbird and other email clients sometimes have a means to write from right-to-left so that in the compose window the message appears correctly for the sender, however the receiver still gets the message with the text aligned at the left hand side, with all the end-of-sentence punctuation backwards.* If there is a single word or term (such as a hyperlink) in Latin letters, the words that appear before that word are moved to the opposite side of the line, so the sentence will be garbled and unintelligible. (The latter is a problem that pops up also in dialogue boxes of the operating system, when the interface language is in a right-to-left language.)
Basically, one is forced to use rich text or html for right to left languages because only there you have direction tags. For email purists, like I try to be, this is an unresolved problem.
"In plain text and Email there are no paragraphs to set the directionality for, and the document directionality is usually LTR. Some text editors and Email editors allow the user to change the apparent directionality on a per-document or a per-line basis. It is important to note that this apparent directionality is not a property of the document, and it will be lost when the file is closed or the Email is sent. It is up to the user who opens the document or the email (or his software) to set the proper apparent directionality."