Postgrey does not launch correctly on boot after upgrade from 14.04 to 16.04

[ilpssun]

I upgraded my Ubuntu 14.04 LTS to 16.04.1 LTS and now I have a problem with my postgrey installation. When I (re)boot the server, the postgrey startup script is apparently called but postgrey does not launch correctly. Only after manually restarting the service is postgrey working correctly.

In more detail: After reboot the following lines appear in my syslog:
Dec 1 19:22:14 systemd[1]: Starting LSB: Start/stop the postgrey daemon...
Dec 1 19:22:14 postgrey[998]: * Starting postfix greylisting daemon postgrey
Dec 1 19:22:15 postgrey[1050]: Process Backgrounded
Dec 1 19:22:15 postgrey[1050]: 2016/12/01-19:22:15 postgrey (type Net::Server::Multiplex) starting! pid(1050)
Dec 1 19:22:15 postgrey[998]: ...done.
Dec 1 19:22:15 systemd[1]: Started LSB: Start/stop the postgrey daemon.

When I then ps for postgrey, no process is found. However, a simple sudo systemctl restart postgrey makes it run properly. The launch is documented in syslog as follows:
Dec 1 19:25:54 systemd[1]: Stopping LSB: Start/stop the postgrey daemon...
Dec 1 19:25:54 postgrey[1878]: * Stopping postfix greylisting daemon postgrey
Dec 1 19:25:54 systemd[1]: postgrey.service: Control process exited, code=exited status=1
Dec 1 19:25:54 systemd[1]: Stopped LSB: Start/stop the postgrey daemon.
Dec 1 19:25:54 systemd[1]: postgrey.service: Unit entered failed state.
Dec 1 19:25:54 systemd[1]: postgrey.service: Failed with result 'exit-code'.
Dec 1 19:25:54 systemd[1]: Starting LSB: Start/stop the postgrey daemon...
Dec 1 19:25:54 postgrey[1886]: * Starting postfix greylisting daemon postgrey
Dec 1 19:25:54 postgrey[1886]: Pid_file "/var/run/postgrey.pid" already exists. Overwriting!
Dec 1 19:25:54 postgrey[1893]: Process Backgrounded
Dec 1 19:25:54 postgrey[1893]: 2016/12/01-19:25:54 postgrey (type Net::Server::Multiplex) starting! pid(1893)
Dec 1 19:25:54 postgrey[1886]: ...done.
Dec 1 19:25:54 systemd[1]: Started LSB: Start/stop the postgrey daemon.
Dec 1 19:25:54 postgrey[1893]: Resolved [localhost]:10023 to [::1]:10023, IPv6
Dec 1 19:25:54 postgrey[1893]: Resolved [localhost]:10023 to [127.0.0.1]:10023, IPv4
Dec 1 19:25:55 postgrey[1893]: Binding to TCP port 10023 on host ::1 with IPv6
Dec 1 19:25:55 postgrey[1893]: Binding to TCP port 10023 on host 127.0.0.1 with IPv4
Dec 1 19:25:55 postgrey[1893]: Setting gid to "120 120"
Dec 1 19:25:55 postgrey[1893]: Setting uid to "112"
As you can see, this time postgrey documents its binding to the correct port. When I look through my ps output now, I can see the working process there.

My suspicion is that the network is not yet ready. If I edit `/etc/init.d/postgrey` to include `$network` in the `Required-start` section, it seems to work correctly. As I'm not experienced with the linux boot process, I'm not sure whether this is a problem with my specific system or a general bug. Also, I'm not sure whether the editing the init script has any side effects that I'm not seeing.