So, when postgrey starts it puts a pid file in the directory /var/run/postgrey. On Debian 9 /var/run is on a temporary file system and is erased on reboot. The default init script for postgrey on does NOT create the directory for its pid file. So then you reboot the host and postgrey doesn't start and postfix returns errors about the mail server not being configured correctly and your mail all bounces for several days. What the hell, Debian.