Comment 24 for bug 1519331

Had the same problem before Zesty and used some of the fixes described here to have postfix restart through rc.local file after waiting a few minutes. The bug is still persistent for me after upgrading to Zesty. What does fix it for me though is adding the "After" line to the Unit: /lib/systemd/system/postfix@.service

After=network-online.target

Note: it did not work to make this change to the postfix.service unit.