Kolab is a pipe dream, it suffers from a lack of documentation, leadership and upgradability. Install it and successfully get it running, with a single domain, then when the time comes to upgrade, you may as well just build a new server. Upgrades break EVERYTHING. Undocumented changes from version to version. Nearly any modern (since about 1996) email server can handle multiple virtual domains...Kolab can't, and there isn't any kind of drive from the project to make it any easier to do. It would cut into the competition for their email service if it were made easy. Stick with iRedmail and get a faster, better email server built from nearly the exact components.