Discussed on Mac Geek Gab 383, as many Mail users know, a quick keyboard shortcut to reply to an email message is Command-R. This will duplicate the effect of hitting the “Reply” button in the Mail toolbar and will open a new mail window addressed to the sender of the message to which you are replying. Alternatively, the keyboard shortcut Command-Shift-R will perform a “Reply All” function, addressing the reply to all visible recipients of the original message.
Scout discovered, however, that you if you change your mind or make a mistake between Reply and Reply All, you don’t need to close the already open New Message window and start over. Simply pressing the alternate keyboard shortcut will adjust the message accordingly, turing a “Reply” into a “Reply All” and vice versa.
For those who are averse to keyboard shortcuts, the same effect can be achieved by pressing the reply / reply all buttons in the New Message toolbar.
The “Reply” and “Reply All” buttons in Mail’s New Message toolbar.
This works on the latest versions of both Snow Leopard and Lion.