The one thing that has annoyed me about Apple Mail is that there are many times I would get an e-mail from random people and the font size of the text would be around 6-8 points. I’d always have to hit cmd + to increase the font size to where I could read it.

Here’s how I solved the problem:

Close Mail and open up terminal and type the following:

defaults write com.apple.mail MinimumHTMLFontSize 13

Hit enter and then launch Mail. Problem solved!