I am hosting multiple domains and have this set up for some of them. It isn't hard, you just need to follow the directions. It is a little too complicated for me to go into here, but below is a link that should get you going. Otherwise google or search here for postfix and multiple domains. Post any errors or problems that you may run into and someone can help you there.
virtual_mailbox_domains = arkum.ch
If you have many domains, you can list them in a file and point virtual_mailbox_domains to the file:
virtual_mailbox_domains = /usr/local/postfix/etc/virtual_domains