set append-new-mail

SET APPEND-NEW-MAIL NO/yes  -  whether to append to mbox or rewrite mbox

With the usual NO setting, when new mail is received, MM renames the file 
  defined by MAIL-FILE (normally "mbox") to the filename with ~ (normally 
  mbox~), and then completely rewrites MAIL-FILE with the new mail included.

If you change it to YES, incoming mail (new messages) will be appended 
  directly to the file defined by MAIL-FILE.  Setting to YES enables MM to 
  store incoming mail more quickly.  However, it is not quite as safe as 
  setting to NO: while there is no more likelihood of losing new mail, the 
  statuses of messages are updated on disk much less frequently.

For more help, type "help" and one of these topics:
(at any prompt:)  check   set mail-file
(at the MM> prompt:)  expunge   write