After running a system upgrade on your Ubuntu based ISPConfig 3 server, it might happen that aptitude overwrites some config files from postfix especially the main.cf file. To fix this, download the latest ISPConfig 3 installer, unpack it and run the update.php script. Choose "yes" when the updater asks if the services shall be reconfigured. ISPConfig will replace the configuration files that had been modified by the ubuntu upgrade.
tar xvfz ISPConfig-3-stable.tar.gz
php -q update.php