Apple Mail, often referred to as Mail.app or simply as Mail, is the built-in mail client for OS X. Since Mac OS X 10.4 (Tiger) it has stored messages in a single file to allow indexing with Spotlight. Coincidentally Tiger was my first real introduction to OS X.
Mac OS X 10.5 introduced, must to my disappointment, 'Stationary' allowing once to come up with many hideous HTML designs for your messages. In this release we also received notes and to-do support. Sadly this is not synced across to the iPhone / iPod Touch.
Force Apple Mail to display the plain text alternative if there is one
Quit Mail.app Open Terminal.app Enter defaults write com.apple.mail PreferPlainText -bool TRUE and press enter Open Mail.app
Specify the minimum font size Apple Mail uses for HTML messages
Quit Mail.app Open Terminal.app Enter defaults write com.apple.mail MinimumHTMLFontSize 13 and press enter Open Mail.app