set gnuemacs-mmail

SET GNUEMACS-MMAIL YES/no  -  whether editor is gnuemacs with mmail support

With the usual YES setting, when you use gnu emacs in mm, you will be put into
  a customized mmail mode.  You will have two emacs windows: the upper one
  contains either the headers of your message (if you are SENDing a message) 
  or the message to which you are REPLYing, and the lower one contains the 
  text you are writing.  (In a REPLY, a third undisplayed buffer contains the
  headers.)

If you change it to NO and use emacs, you will be put into emacs fundamental 
  mode, with one window.  

Set it to NO if you do not want the split window support.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   set editor