expunge

EXPUNGE  -  permanently remove messages marked as deleted

Usage:  EXPUNGE

The EXPUNGE command rewrites the current mail file, omitting all messages
  marked as deleted by the DELETE or MOVE commands.  As a result those
  messages are permanently removed, and the mail file is smaller.  Use
  EXPUNGE to clean up after you have used DELETE and MOVE.

Leaving MM with the EXIT command automatically does an EXPUNGE on the
  current mail file.

For more help type "help" and one of these topics:
  delete   undelete   exit   quit   bye   headers   move   filing