You have a mailbox you need to sort. You can't have it come back through your MTA, of course. Use this script to push it through the procmail filter of your choice:

#!/bin/sh

ORGMAIL=/var/spool/mail/$LOGNAME

if cd $HOME &&
  test -s $ORGMAIL &&
  lockfile -r0 -l3600 .newmail.lock 2>/dev/null
then
  trap "rm -f .newmail.lock" 1 2 3 15
  umask 077
  lockfile -l3600 -ml
  cat $ORGMAIL >>.newmail &&
  cat /dev/null >$ORGMAIL
  lockfile -mu
  formail -s procmail <.newmail &&
  rm -f .newmail
  rm -f .newmail.lock
fi
exit 0