$ sudo pkg install procmail
$ man formail

$ mkdir ~/work
$ cd ~/work

$ cat << EOF > cat-msg.sh
#!/bin/sh

cat > msg-$FILENO.txt
EOF
$ chmod 755 cat-msg.sh

$ cat << EOF > split-mbox.sh
#!/bin/sh

export FILENO="00000"
formail -s cat-msg.sh < "$@"
EOF
$ chmod 755 split-mbox.sh

$ sudo service sendmail stop

./split-mbox.sh /var/mail/$USER

$ grep -lF '^From: spammer@example.com' msg*.txt
$ (use less to spot-check a few or all of them)

$ grep -lF '^From: spammer@example.com' msg*.txt | xargs rm -v

$ cp -vp /var/mail/$USER mbox-$USER
$ cat msg*.txt > /var/mail/$USER
$ sudo service sendmail start

$ sudo mv -vi split-mbox.sh cat-msg.sh /usr/local/bin/