hi,
there is a small error in the emacs manual in chapter Sending mail :

C-x 4 m
Likewise, in another window (compose-mail-other-window).
C-x 5 m
Likewise, but in a new frame (compose-mail-other-frame).

instead

C-x 4 m
Likewise, but in a new frame (compose-mail-other-frame).
C-x 5 m
Likewise, in another window (compose-mail-other-window).