I've just posted an enhancement in the Bugzilla for an instant reply function in an email reading window.
It basically is a little text entry field and a send button at the bottom of the email reading window that you can enter very short responses into and press the send button. When you do it takes you back to the inbox and sends a reply email just as if you had pressed reply, typed your response and pressed send.
It doesn't seem to be much of a feature that does anything but I used to use an email client that offered this and it was the most useful thing ever!! I think it was called Barca or something like that.
There are more details in the bug description but if you think it's a worthy feature please go and vote for it. It's located here...