I get email which seems properly plaintext formatted, but when I forward it out, the font size ends up being enormous and a few other formatting stupidities. So I tried the following:

defaults write com.apple.mail PreferPlainText -bool TRUE

And this works great, all outgoing email regardless of the text original is plaintext only, just the way I like it. But now for incoming email only the plaintext portions are available to read without going into View -> Message -> Raw Source (or something similar to that).

Does anyone know how I can force only outgoing mail to include only plaintext, but continue to read incoming mail in the formatting it was intended?