Opposing counsel sends you an e-mail, copying his client on it. Can you "reply to all" without violating rule 4.2, which prohibits communications with a person a lawyer knows to be represented by counsel in a matter without the consent of the opposing lawyer?
Maybe. That's what the New York City Bar Association reasoned in opinion 2009-1, available here. Even if it is appropriate to "reply to all," the opinion warns that implied consent beyond that time and subject matter likely should not be implied.
I've seen other cases which reject implied consent under Rule 4.2, so be careful out there!