postrm is in mail-stack-delivery 1:2.2.22-1ubuntu2.
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 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 | #!/bin/sh
set -e
POSTFIX_BCKFILE="/var/backups/mail-stack-delivery/main.cf-backup"
PATH=/usr/sbin:$PATH
export PATH
if [ "$1" = "remove" -o "$1" = "purge" ]; then
	# Restore postfix configuration
	if [ "$1" = "remove" ]; then
		if which postconf >/dev/null && [ -f "${POSTFIX_BCKFILE}" ]; then
			while read line; do
				postconf -e "$line"
			done < "${POSTFIX_BCKFILE}"
			rm -f "${POSTFIX_BCKFILE}"
		fi
	fi
    if [ -x "/etc/init.d/dovecot" ]; then
        if [ -x /usr/sbin/invoke-rc.d ]; then
            invoke-rc.d dovecot restart
        else
            service dovecot restart 
        fi
    fi
	if [ -x "/etc/init.d/postfix" ]; then
       	if [ -x /usr/sbin/invoke-rc.d ]; then
            invoke-rc.d postfix restart
       	else
            service postfix restart
       	fi
	fi
fi
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