set editor

SET EDITOR [program]  -  define the editor
Usually:   /usr/local/bin/emacs

The EDITOR variable defines what editor is used when you use the EDIT
  command or use control-e while writing a message.  The usual setting
  defines gnu emacs as the editor.  

If you use another editor, probably you want GNUEMACS-MMAIL set to NO.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   edit   set control-e-editor
                  set gnuemacs-mmail   set use-editor-always
(at the MM> prompt:)  send