You know those locked plastic covers they have over the Big Red Button in movies, regardless of what that Big Red Button does? They need that same thing over the "Reply All" button in email programs. That action needs to have some real gravitas. Not a pop-up asking "Are you sure?", but the action itself impressing upon you that this is a big thing you are doing.
Mokalus of Borg
PS - It's going to be a fair bit more annoying.
PPS - And that's kind of the whole point.