How to relay E-Mails to "mydestination" for users with no local Mailbox

[Martin Klewitz [gamigo AG]]

Hello,

In my environment there are two mailservers. The first (server1) is running 
exim and is pointed by all MX-Records, the second one (server2) is running 
kolab/postfix and receives mails only from local clients and server1. It is 
configured to use server1 as a smarthost.
Both servers handle emails for mydomain.com. Some mailboxes are located at 
server 1, some on server 2.
My problem occurs when user1 at mydomain.com tries to send a mail to 
user2 at mydomain.com over server2 (the mailbox of user1 is located at 
server1):
Postfix on server2 says "Mailbox unknown".

How do I have to configure postfix on server2 to send all mails through the 
smarthost server1 even if the domain is mydestination if a local 
kolab-account does not exist?

Regards,
Martin Klewitz