/usr/share/doc/checkpw/etc/pop/run is in checkpw 1.02-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/bin/sh
exec softlimit -m 4000000 tcpserver -vRH 0 110 \
/var/qmail/bin/qmail-popup `cat /var/qmail/control/me` \
checkpw sh -c '
echo "login: $USER $TCPREMOTEIP" >&7
if [ -n "$EXT" ]; then DASH="-"; fi
exec /var/qmail/bin/qmail-pop3d Maildir"$DASH""$EXT"
' 2>&1 7>&1
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