I've got email notification activated for all incoming messages so I won't miss any. I love the feature that I directly can respond to that email to write to the user, thats great!
However, it seems that messages which have been written that way via mail, don't get counted to the responsiveness value. I read the according FAQ entry, and as far as I understood it, it doesn't say anything about this.
From how I understand the feature of responding via the "reply" function of that mail notification, it will be processed on your hosts anyway, depending on the reference code in the headline. I think therefore it should also be possible to notice, that there was a response.
Actually, it also would be great, if my mail-response would show up in the message-conversation, too. Maybe this way it would solve the responsiveness issue already. Its a pity my response quote is at only 50%, I try to answer all messages I receive.