The documentation says to use aptitude -y install kolab while aptitude is not installed by default on a clean Ubuntu system. So, the doc should ask for checking this or just propose to install aptitude with apt-get install aptitude.
Instead, I tried to install kolab with apt-get install kolab but it failed with dependency issue on kolab-webadmin.
When using aptitude way it works. It may make sense to note in documentation that when installing postfix it asks some questions. Leaving default choices should be ok.