Exim3 Route to Zimbra

[quietas]

I have an existing server running Exim3. I've followed the Split Domain wiki instructions. How do I do the following?

Make sure to configure mail.example.com to
* accept mail relayed by zimbra.example.com.
* forward mail to zimbra.example.com for accounts hosted on zimbra.

Any ideas on what I need to edit to enable the Exim server to forward//route messages to the Zimbra box?

[uxbod]

Are you wishing to leave Exim as the front-end (MX) MTA to receive the email and then forward onward bound to ZCS ?

[quietas]

The existing server is staying there as is for the time being. Exim 3 stays unfortunately.

Also, do I need an MX record on our DNS servers since the primary server will route messages? I never actually anthing that said one way or the other.