If you have problems with HTML emails received in OS X Mail, you can force the Mail app to display e-mails in plain text using the following defaults write command.
defaults write com.apple.mail PreferPlainText -bool true
You need to restart Mail for the changes to take effect.
Restore to default with:
defaults write com.apple.mail PreferPlainText -bool false
This has no effect in Mountain Lion 10.8.2
It does for me in 10.8.2. It seems necessary to run it while mail is not running.
Does not work for me in 10.8.4