set suspend-on-quit

SET SUSPEND-ON-QUIT YES/no  -  whether the QUIT command suspends

With the usual YES setting, the Top-Level QUIT command suspends the 
  MM process.  You return to the shell prompt, but the MM process still 
  exists and you can return to it quickly with a fg command (the "mm"
  command is aliased to the command "fg -s mm", meaning fg or start).

If you change it to NO, QUIT kills the process.  To run MM again, you
  would have to start a new MM process.  See the BYE command.

This variable is not related to the Read or Send Mode QUIT commands.

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