Displaying plain text by default in mail.app
Most email you receive probably appears in fancy fonts with colours and backgrounds (it's HTML, just like a web page) and this can be hard to read. If you use the Mail.app for your email you can press Command-Option-P, or select View->Message->Plain Text Alternativefrom the menu bar, and if the email has a plain text version available it will display that version. If you want the fancy version back, press Command-Option-H, or select View->Message->Best Alternative from the menu bar.
Most HTML email does have a plain text version, in my experience the only ones without are spam and some newsletters. You can configure this text version to display by default with the following steps:
Quit the Mail.app program if it's running. Open Terminal.app, in the /Applications/Utilities folder, and then type this command, followed by the Return key:
defaults write com.apple.mail PreferPlainText -bool TRUE
Now you can start Mail again, and messages that have a plain text version will display that by default. If you want to see the fancy HTML version of an email, you can still use the Command-Option-H or View->Message->Best Alternative commands.
To set the default back to seeing the HTML version, use the same instructions as above, but change the line to:
defaults write com.apple.mail PreferPlainText -bool FALSE