From Kolab wiki

Using multiple domains in Kolab

This is aimed at users of versions prior to 2.1. Version 2.1 and above include multi-domain support.

eMails

Edit /kolab/etc/kolab/templates/main.cf.template and add your extra domain onto mydestination: Just change the line:

 mydestination = @@@postfix-mydestination|join @@@

to

 mydestination = @@@postfix-mydestination|join @@@ myotherdomain.com

You will also need to add aliases to the users who you want to receive mail for the other domains.

You can also make groups (Distribution Lists) receive mail for these domains by adding another mail attribute to them using your favourite ldap editor (This worked for me but may beat you up and steal your lunch, research this before commiting to it!)

Websites

--coming soon--