If you want to reply to an email and you just want to include part of the original message in your reply instead of the entire message, highlight the portion you want to include and then hit the Reply button. It'll only include the highlighted portion as the "original" email in your reply. Seems like this'll be handy for getting ridding of the long email chains that sometimes occur.

I stumbled across this in Mail client on the Mac, not sure if it'll apply to other email clients like Outlook or not. But it's worth a shot.