How to default UTF-8 encoding in Text Encoding in Mail.app?
One way to ensure all your email goes out as UTF-8 is to include a Unicode dingbat or similar character in every message, as part of your signature or otherwise.
Open Terminal.app and type:
defaults write com.apple.mail NSPreferredMailCharset "UTF-8"
If it does not work, you can revert by using
defaults delete com.apple.mail NSPreferredMailCharset