You can freely mix Arabic text with English or any other languages of left-to-right flow direction. This direction actually has nothing to do with alignment.
The only problem is editing such texts, because the actual behavior of text caret depends on the character repertoire
of the neighboring characters. Even though it almost drove me crazy, in all cases I managed to type required text (anyway, please don't talk to me in Arabic: unfortunately, I cannot understand the language itself, I remember only very few words — I did it only for testing some software
If you still have any concerns about alignment, you should understand that it has nothing specific to e-mail. Rather, it depends on the content type. Basically, you have a choice from the content type "text/plain" or "text/HTML". In case of plain text, there is nothing like real alignment. Just try to mix the right-to-left and left-to-right text and see how it looks: to me, quite readable and even editable. With HTML, you can control alignment with CSS. When specifying content type, don't forget to specify encoding. Most typically, it should be UTF-8; and this is most advisable, too: "text/plain; charset=utf-8" or "text/html; charset=utf-8".
Of course, you always can write multi-part e-mail message, then each part can have its own content type.