The scourge of the 'Reply All' button

I somehow was put onto a mailing list from a company I don't know and, as of this writing, I have been copied on at least 20 'please unsubscribe me' messages from people all over the country.  While this doesn't happen too often, it got me to thinking of a feature I'd like to see in email clients.  In keeping with good email practice of not hitting Reply All without a really good reason to do so, perhaps there could be a rule built in to email clients that would provide a pop-up whenever senders click Reply All.  The popup would remind people of the danger of clicking Reply All, particularly to a large list of recipients.  The popup could be customized by administrators and could be turned off only temporarily (maybe after every 10 Reply Alls the popup would reappear).

In the grand scheme of things, this would solve a relatively minor problem, but would be useful nonetheless.