As an update of sorts, we just had someone file a grievance because someone higher-up an email chain was careless and didn't excise remarks concerning the person in question, and they subsequently found their way into that person's inbox through this method. (The initial emails were between management discussing this person's conduct and performance, and the conversation continued and continued, and eventually someone was told "Okay, we need to send an email to Person X encouraging them to improve X, Y and Z", and so one was written--but the author of this message was careless and didn't bother excising the earlier discussions from this new message.)
Clear your email trees, guys. It's the right thing to do. If you MUST MUST MUST keep your conversations in a single place, get a gmail account.