postrm is in squirrelmail-sent-confirmation 1.6-2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18  | #!/bin/sh -e
set -e
# Source debconf library.
case "$1" in
    remove)
	if [ `which squirrelmail-configure` ]
	then
	    squirrelmail-configure --remove-plugin sent_confirmation
	fi
    ;;
    *)
    ;;
esac
exit 0
 |