prerm is in exim4-daemon-heavy 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.
| 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 | #!/bin/sh
set -e
if [ -n "$EX4DEBUG" ]; then
  echo "now debugging $0 $@"
  set -x
fi
case "$1" in
	remove|upgrade)
    	if [ -x /etc/init.d/exim4 ]; then
		if [ -n "$EX4DEBUG" ]; then
		  netstat -tulpen
		  ls -al /run/exim4/
		  cat /run/exim4/exim.pid
		  pidof exim4
		fi
		invoke-rc.d exim4 stop
		if [ -n "$EX4DEBUG" ]; then
		  netstat -tulpen
		  ls -al /run/exim4/
		  cat /run/exim4/exim.pid
		  pidof exim4
		  if pidof exim4; then
		    echo >&2 "WARN: There are some exim4 processes still running after stopping exim"
		  fi
		fi
	fi
	;;
esac
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