Let’s say you send out an email to several people, and a recipient does a Reply-All to say something, but adds yet another recipient at the same time. Would you notice? Would you want to notice, so you don’t accidentally say something you shouldn’t?
Gmail highlights these new recipients in bold. Presumably it inspects the previous emails in the “conversation” it’s a part of, and notices that some new people have joined the discussion. An innovative idea that solves a problem that I never even realized as a problem.
Now, if only they could do something about the accidental Reply-All…
Advertisement