set directory-folders

SET DIRECTORY-FOLDERS NO/yes  -  whether to add a slash with mh format

With the usual NO setting, MM assumes that a mail file is a file containing
  one or more messages.  This is correct for formats mbox, babyl and mtxt.

** MM DOES NOT YET SUPPORT MH FORMAT, SO DIRECTORY-FOLDERS SHOULD ALWAYS
   BE SET TO NO **

If you change it to YES, MM assumes that a mail file is what is known in
  mh format as a "folder".  mh keeps each message as a separate file in
  a directory called a "folder" that corresponds to what would be a single 
  file in other mail formats.  This variable controls how slashes are
  added to complete directory and file names when you use TAB to complete
  a name.

For more information on MH, type "!man mh" at an MM prompt, or "man mh"
  at the shell prompt $.

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