I'm in early stages of migrating exchange to zimbra,

before migrating, i have two exchange servers with one domain.
one in HQ and one in our branch office.
If user in branch want to send an email to user in HQ,

For example, john@office.com in branch A office want to send an email to jane@office.com in HQ. (exactly the same email domain name).

The exchange server will look if jane is exist in local server (branch A server), if exist the mail would be delivered to jane's mailbox, in this case jane is in HQ, so exchange will deliver the mail to HQ server.

We use internet mail service in exchange connection.

When we try this with zimbra, all email sent from branch to HQ bouncing back, because zimbra does not look for if user exist, if it's not exist then relay to HQ server.
Zimbra just look the domain name, and try to find the user in local server and that's it.

How can i configure zimbra so, it can act like what exchange do before.

Thank you.