On the Mac Mail program there is a preference that allows you to set your quoting behavior when you reply to an email. Go to your preferences (Mail -> Preferences...) and select the Composing tab. At the bottom of the panel is an entry called "When quoting text in replies or forwards:" and there are two options. The first is "Include all of the original message" and the second is "Include selected text if any, otherwise include all".

If you choose the second option you can highlight any text you want in the original message. Then when you reply or forward only the selected text will be including in the quote of the original message. If you don't select anything, the entire message will be included which is the normal behavior. Having this option on makes it a lot easier to reply to a specific portion of an email.