set handle-changed-modtime

SET HANDLE-CHANGED-MODTIME ALWAYS/never/ask  -  how MM handles a file problem

The HANDLE-CHANGED-MODTIME variable controls what MM does when it appears
  that some other program has changed the current mail file.  Whenever you 
  GET a file, MM locks the file (preventing another MM process from 
  modifying it), and then while the file is locked MM keeps track of
  the last time it modified the file.

With the usual ALWAYS (or YES) setting, if MM finds that the last time the 
  file was modified does not match, it writes the version it has in memory 
  to a new file, and EXAMINES that file (that is, you have read-only access 
  to it).  The purpose is to let you decide which version of the mail file 
  you want, and to stop incoming mail until you have a chance to look into 
  the problem.  

If it is set to NO, MM will just print a warning when the condition occurs.
  If it is set to ASK, MM will ask whether to follow the YES procedure.

For more help, type "help" and one of these topics:
(at any prompt:)  save-init   set modify-read-only
(at the MM> prompt:)  get   examine