If you have multiple domains that you want to use on Zimbra, but want to use the same mailboxes than Domain Aliases are your answer. To setup a domain alias, do the following:
- In the client area, go to Services -> My Services and click on the Zimbra service
- NOTE: If you have multiple Zimbra services, pick the plan that you want to add to
- Click "Domain aliases"
- Enter the domain in to the New Domain Alias form
- Pick which domain to alias
- Submit the form
You should be set at this point.
NOTE: If you do not see Domain aliases, your Zimbra plan may not offer this and an upgrade would be needed.