In article
<debcded4-2e70-4f10-9f01-6d01c0efa318@z17g2000hsg.googlegroups.com>,
> How can I tell Apple Mail to send every new message with UTF-8 as text
> encoding instead of the automatic setting which causes problems under
> 10.5.2 Leopard? The manual setting switches back to automatic with the
> next new mail...
I think you're looking for this command (in Terminal):
defaults write com.apple.mail NSPreferredMailCharset "UTF-8"
See also,
http://www.macosxhints.com/article.php?story=20040906175135489
Hope it works.