postinst is in exim4-daemon-heavy 4.76-3ubuntu3.
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
. /usr/share/debconf/confmodule
if [ -n "$EX4DEBUG" ]; then
  echo "now debugging $0 $@"
  set -x
fi
dc_eximconfig_configtype="other"
db_get exim4/dc_eximconfig_configtype || true
if [ "$RET" = "no configuration at this time" ]; then
  dc_eximconfig_configtype="none"
fi
case "$1" in
	configure)
	# || true is needed for succesfull installation with configtype 'none'
	if [ -x /etc/init.d/exim4 ]; then
		db_stop
		if [ "$dc_eximconfig_configtype" = "none" ]; then
			# we may have broken config here, ignore errors
	       		invoke-rc.d exim4 start || true
		else
			# we must have working config here, honor errors
			invoke-rc.d exim4 start
		fi
	fi
	;;
esac
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