You can use the following trick to force Mail to always use a larger minimum font size on received messages - some HTML messages might not look exactly as intended but at least you can now read them:
1. Quit Mail
2. Open the Terminal and type in the following:
defaults write com.apple.mail MinimumHTMLFontSize 13
3. Restart Mail.app and your HTML messages are readable.
Of course you can set the point size to whatever you like.
Andreas
PB G4 Al 1.5GHz/1.25GB, PB G4 Ti 550MHz/768MB, iPod 10GB (2. Gen), AEBS & AEX Mac OS X (10.4.5) http://homepage.mac.com/aamann/