A little back story for those wondering why:
I am in charge of getting a new Zimbra server up and running. My company currently outsources email hosting to another local company. This company has a proprietary email server that all of our customers are on. The email for all of our employees is through GMail, but it's with OUR domain. email.myco.com is where the employees go to get their email, which takes you to GMail and login with our usernames, example@myco.com. mail.myco.com is where the customers go and they're greeted with the proprietary server frontend.

So my question is, how does on accomplish this? How do I tell the zimbra server to forward all email coming in to x, y, z accounts out to the Google server, but keep all emails for accounts a-v? I thought about forwarding email from each account, but I don't think that will work since the domain remains myco.com on ALL accounts, customers and employees.

For the record, I did search the forum for this for about 2 hours, but all I could come up with is a single account forwarding from one email address on one domain to another email address on another domain.