We recently got into ubuntu autoinstall. I planned to install postfix via packages and configure the main.cf with sed in late-commands section.
Since postfix installation is interactive I wanted to use debconf-set-selection like described here: Automate the installation of postfix on Ubuntu
But I think the apt-get install postfix collides with the package section install of postfix.
Maybe I am able to achieve the debconf changes with the debconf-selections section for the autoinstall, but the information: "Users will need to be familiar with the package debconf options." does not really give me the right hint how to set it up.