Symptom: Every email I send from Apple Mail looks good as far as Apple users are concerned. Any Apple Mail client will display it correctly. However, Windows Outlook users will see a bunch of weird characters (squares, question marks, etc) in my emails.
Reason: Mail should be set to always use Unicode as the default encoding for new emails. This can of course be done manually from the Message » Encoding menu when typing an email and before sending it, but it's a pain to do it manually, and there's no menu option nor preference to set this as default
Solution: Open Terminal and login as root. Then execute the following command to set Unicode as the default encoding:
defaults write com.apple.mail NSPreferredMailCharset "UTF-8"
Restart Mail and try again.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070808165831835