I don't specifically know what is common in the way of non-Unicode encodings of Arabic text. The essential thing about email is that each email message is generally written with an ASCII or UTF-8 encoding, and that anything that is not in that format needs to be wrapped as a MIME "attachment" block. The email client then inspects those MIME headers to make sense of your message.

You might want to see my FMTYEWTK about Characters vs Bytes for a generic primer to Unicode (and how Unicode itself isn't exactly an encoding).