Hi Zimbra-ites

On my zimbra server I have several domains. Lately I've been seeing spam coming from a non-existent account on one of my internal domains. All the messages are coming from voice*@mydomain.com where * = a random number.

I'd like to block this spam without affecting legitimate users that do have accounts on mydomain.com. Is this possible? I was thinking something like block all mail from random-user@palmettoltc.com if random-user does not have an actual account.

I am also open to other suggestions. The above was just the first idea that popped into my head.

Let me know if any more details are required.
Thanks