If delivering to a single secondary mail system, configure a catchall on the domain to deliver mail to a secondary mail transport if users for the same domain reside on the second system.

1. zmpov cd example.com
2. zmprov md example.com zimbraMailCatchAllAddress @example.com zimbraMailCatchAllForwardingAddress @example.com zimbraMailTransport smtp:mail.example.com:25

Mail for Zimbra recipients on the primary system to @example.com will be delivered Zimbra mailboxes. Any address that does not exist on the Zimbra system will be routed to the secondary mail transport.

Reference:

http://wiki.zimbra.com/index.php?title=Split_Domain