By default, when you’re replying to an email using Mac Mail,  all previous emails will also be included below your response. In certain cases, that’s a good thing, as you want the other party to see all previous correspondence. But sometimes, you just end up with overly long and messy looking email threads.

There’s an easy way to remedy this, by changing one setting in your Mail preferences. By choosing Mail > Preferences > Composing you will find a couple of relevant options under the Responding area. If you choose the option to “Include selected text, if any; otherwise include all text”, you can now simply select an area of the email you want to respond to, and only that selection will be included in your reply email. This allows for much more clutter-free back and forth replies.
Experiment with it, and you’ll see what I mean.

Mail reply settings