postrm is in exim4 4.90.1-1ubuntu1.
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.
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#!/bin/sh set -e # Automatically added by dh_installdebconf/11.1.4ubuntu1 if [ "$1" = purge ] && [ -e /usr/share/debconf/confmodule ]; then . /usr/share/debconf/confmodule db_purge fi # End automatically added section