Can't send mail to shared folders: kolab+shared.archiv at domain.tld

[j.c.f.]

<snip>
> Am Donnerstag, 13. Oktober 2005 16:50 schrieb Alexander Maret-Huskinson:
> > Am Dienstag, 11. Oktober 2005 14:32 schrieb Alexander Maret-Huskinson:
> > > I'm trying to debug my problem of not being able to send mails to
> > > shared folders. Postfix rejects all mails sent to
> > > 'kolab+shared.<foldername>@domain.tld' with 'User unknown'.
</snip>

I solved the problem with those 2 lines in main.cf, but I am not quite sure 
why it worked out, any hints?
virtual_mailbox_maps = $virtual_maps
main.cf.template:local_recipient_maps = $virtual_maps

Joe Karma