Got Mojibake in Here's a Cure.

If you use OS X Mail to send Japanese email, you may find that some recipients complain that your Japanese is “mojibake” or garbled. If you are getting this feedback you can set your default Character Set encoding in the terminal.

How to Set’s Default Charset

Here’s how to change it. Shut down mail, then in Terminal:

$ defaults write NSPreferredMailCharset "ISO-2022-JP"

That will set Mail’s default character set to ISO-2022-JP instead of UTF-8, and after you restart, you will be golden. I tested this in 4.2 in OS X 10.6.3 and it works for me (at least according to my Outlook-saddled colleagues).