You can also do something like this:
miniGeek.org Postfix archive emails via bcc
It'll separate the emails out by user rather than dumping them all into one account. You will probably want to add the hash file as the blog post suggests rather than adding a field to Zimbra's LDAP. The post isn't Zimbra specific, but since Zimbra uses Postfix it's all the same. Like the previous post says, you'll have to re-apply changes after an upgrade.
You wouldn't have to necessarily make sure it's a local account. You could do it to another mail server, but make sure to send it to one you own so you don't get yourself blacklisted.