When a program (or a person) wants to respond to a message—reply,
follow up, forward, cancel—the program (or person) should just put
point in the buffer where the message is and call the required command.
Message will then pop up a new message mode buffer with
appropriate headers filled out, and the user can edit the message before
sending it.
You can customize the Message Mode tool bar, see M-x customize-apropos RET message-tool-bar. This feature is only available in Emacs.