Hm, I don't think this is the situation I'm talking about. This document assumes you have 1 domain (example.com) and some of the users are on Zimbra and some are still on another mail server.
In my case I have 2 domains: example.com and domain.com. example.com is on Zimbra and running. I want to add domain.com to Zimbra, but any mail for domain.com should go to the existing domain.com mail server.
Here's what I'm thinking might work.
Create the second domain through the admin console.
Then follow the instructions at:
Managing Domains - Zimbra :: Wiki
I've pasted these instructions below. But I want to know in advance whether this would be the proper approach. Also the other mail server is not a zimbra server. But I don't see why that would matter.
"Relaying/Domain Forwarding
If you want the Zimbra server to relay all mail destined for a particular domain (say, example.com) to another mta (say, other-mta.domain.com), you can forward the domain.
zmprov
md example.com zimbraMailCatchAllAddress @example.com
md example.com zimbraMailCatchAllForwardingAddress @example.com
md example.com zimbraMailTransport smtp

ther-mta.domain.com"