Postfix accepting mail for subdomains

[Alex Kent]

Hello,

When I send mail via my kolab 2.2 server (kolab.domain.tld) to a server 
(lists.domain.tld) that isn't in kolab's list of domains specified via 
the web client, the kolab server accepts the mail and then bounces it 
when no corresponding user is found.

Could this be a problem with kolab automatically accepting mail for 
subdomains of domain.tld?

If so, how do I fix?

Thanks,

Alex