I'm using Mail.app.
Often times I would write on reply emails typed in foreign language (Chinese, Japanese), and the receiver will see garbled text (e.g. a bunch of ??? or squares).
The way I solve the problem is: before sending each of these reply email, I go to "Message" -> "Text Encoding" -> select "Unicode *UTF-8*". But as soon as the email is sent, text encoding is set back to "Automatic". On my next email, I have to set the text encoding again to UTF-8.
This is annoying. Is there a way to set UTF-8 as default text-encoding, and not Automatic?