When receiving e-mails containing HTML code in OS X Mail, you will notice that some e-mail messages comes with very small text. With this default write command it’s possible to set the minimum font size for text in HTML e-mails. In some cases changing the size of text will break the layout.
Use the following command:
defaults write com.apple.mail MinimumHTMLFontSize 12
Show current font size:
defaults read com.apple.mail MinimumHTMLFontSize
0 = default
Revert to the default setting with:
defaults delete com.apple.mail MinimumHTMLFontSize