$ cat /etc/X11/xinit/xserverrc
#!/bin/sh

#exec /usr/bin/X -nolisten tcp "$@"
exec /usr/bin/X tcp "$@"

scott@xserver:~$ cat /etc/gdm/custom.conf 
[security]
DisallowTCP=false

scott@xserver:~$ xhost + xclient
scott@xserver:~$ ssh postfix@xclient
postfix@xclient's password: *******

postfix@xclient:~$ export DISPLAY=xserver:0
postfix@xclient:~$