Kerio Connect & Exchange 2003

We are going to be moving from Exchange 2003 to Kerio Connect.  During the migration from Exchange to Kerio I need to keep both servers running until I have successfully move over each user mailbox.  The plan is to move over a few mail boxes at a time, verify they work and then move to the next.  I know that Kerio has an Exchange Migration Tool  but I wasn't real happy with the results when I was testing it.  So I will just export each user's mail to a PDF an then restore to their Kerio Mailbox.  I only have about 40 user's so it not a big deal on the time.  Ideally I would create the users mailbox on Kerio, import the mail, then disable/delete the user's mailbox on Exchange.  With this I would need Exchange to be able to send the users mail from Exchange to Kerio Connect.  Is there a way to tell exchange to send email that can't find a user mailbox to the Kerio Connect Server.  I did find a Messages tab on the properties on Default SMTP Virtual Server that has "Forward all mail with unresolved recipients to host:  " .  Not sure if this is the correct approach or not.  The ultimate goal is to eliminate the Exchange server once all the mailbox are created and the mail has been imported into Kerio.