This is a problem that was introduced with the latest postfix update in the portage tree. The original install instructions required you to add
mail-mta/postfix ~x86
while the latest change now requires you to set
=mail-mta/postfix-2.2.10-r20 ~x86
While this was fixed in the installation instructions I realize that it was quite easy to miss that change. There is a need for a central mailing list that provides announcements and keeps track of changes for the different web applications that are currently provided for Kolab/Gentoo.
Will hopefully be fixed soon.