Do HTML messages sent from Outlook users appear too tiny in your Apple Mail inbox? 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 12
3. Restart Mail.app and your HTML messages are readable.

Of course, you can set the point size to whatever you like.