As you may know, there is a problem with domain aliases and backscattering, that make them unusable and dangerous in a public mail server deployment. This was known and reported four years ago (at the very least) and it still applies to the current release.
The problem is that zimbraMailCatchAllForwardingAddress makes Zimbra accept mail for *any* email address on the domain alias. Only then will it bounce the email back if the user doesn't exist on the destination domain.
Bouncing an email (as in: sending a Delivery Status Notification) to a remote address, when the original destination address didn't even exist, is a practice called "backscattering" and is punished by inclusion in various RBL services.
Domain aliases (by which I mean: accepting email for company.com, company.net, and company.de) is a somewhat common practice, and the only way to make it work correctly in Zimbra, without generating backscatter, is to create specific aliases for all the accounts in the domain, both for current and future users.
This is tedious and error-prone. Does Zimbra have or plan to implement a better solution to this problem?