Some of our customers don't want us to hold mail for them, but rather want us to scan their incoming mail for spam and viruses using our Barracuda appliances, then forward it on to their own mail server. In this case we would relay the mail from the spam appliance to the Zimbra server, and then directly to the client's mail server.
There would be no local mailboxes for this domain on the Zimbra server, simply a queue.
If the client's mail server is not available for any reason, then we need to queue the mail on the Zimbra server until their server is back online.
Can this be done using Zimbra?