UTF-8 Encoding Option Does not Stick

My colleagues who use Outlook on PCs kep asking why my emails have strange text characters. I have done every recommended step to rectify:

via

http://homepage.mac.com/thgewecke/woutlook.html

issuing at Terminal

defaults write com.apple.mail NSPreferredMailCharset "UTF-8"

and setting composing preferences to use Rich Text and same format in replies.

In my mail.com.plist file I can see that NSPreferredMailCharset is correctly set to "UTF=8".

YEST, no matter what I do, on outgoing mail, the portion I write in Mail.app has charset="US-ASCII" not "UTF-8" and the ONLY way to do this is to manually select UTF-8 as text encoding per message.

This is a major PITA and I am clueless why these settings are being ignored and Mail.app is doing whatever the heck it wants to do!