As far as I can tell, we're hitting multiple issues : - before #873957, /lib/systemd/system-generators/postfix-instance-generator "if" could never have matched at all but the else saved us - after #873957, /lib/systemd/system-generators/postfix-instance-generator "if" started working, but using postmulti at this point seems to be a bad idea (some network interfaces/ip might not be up in early boot stages) - systemd not throwing any error when manually starting postfix. This might be related to how debian (badly ?) designed postfix launch sequence for systemd (/etc/systemd/system/multi-user.target.wants/postfix.service starting /bin/true ?) - systemd not logging the failure of /lib/systemd/system-generators/postfix-instance-generator. This might be fixed by intercepting postmulti exit code.
Is generating postfix systemd startup files on each boot a good idea at all ? If you need multiple instances, you should configure them once and for all. Depending on a boot-time script auto-generating which instances are going to be started feels like a very bad idea.
