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.