I have this quite common situation in which I have a customer that has a Postifx mail server (smtp.example.com) and we have to gradually migrate to Zimbra (zimbra.example.com). So, I've set up a Zimbra server on Ubuntu 6 and now I am trying to configure the transport_maps appropriately. Both machines are on the same subnet. My objectives are:
1. Configure Postfix in the existing mail server with transport_maps = hash:/etc/postfix/transports_zimbra and add there the list of email addresses that are being migrated to Zimbra, as follows:
2. Configure Postfix in the existing mail server to allow relay from the Zimbra machine by adding Zimbra's IP to mynetworks.
3. Configure Zimbra to use the existing mail server to send mail.
4. Make all migrated clients use the Zimbra server only (both IMAP and SMTP). Client does not want "half migrated" accounts that use the current mail server as SMTP and the new Zimbra as IMAP.
5. Configure transport_maps in Zimbra in the opposite way, so that all mail for local email accounts (migrated accounts) stay and rest is sent to the existing mail server.
Now I have already done steps 1 to 4. I have read the "Split Domain" wiki article  and I am quite positive that the situation described in "Configuring Zimbra as the Secondary System" should work for me. But the thing is that I'd rather "do it myself" to be sure about what is happening (and to learn, and to be able to undo things in the future - the article does not describe how to undo the steps when the migration is complete).
So, I am trying to give this option to the customer when migrating firstname.lastname@example.org account:
a) Edit /etc/postfix/transports_zimbra in the existing mail server and add this line:
b) Edit /opt/zimbra/postfix/conf/transports:
Also, at the bottom, we would have this line, but just once:
Finally, my problem is that I don't know which is the proper configuration for "transport:nexthop". I have tried with local and virtual, but do not work. I know that Zimbra uses virtual, so I thought that "virtual:[example.com]" should work, but it does not.
Any help will be appreciated. Thanks in advance.
 Split Domain - Zimbra :: Wiki