I just wrote a posting about using Spam Arrest with Zimbra and a Blackberry. I was getting nearly 100 spam emails per day, and now it all stopped. If Zimbra could integrate something like this, or at least work on a Zimlet (even if it was for a fee), it would allow people to really get aggresive about Spam.
One of the key feature of Zimbra is that it pushes to a mobile device, but spam really kills that experience, and Spam Arrest takes the feature away. The Zimbra market is the right demographic for this.
I know not everyone will want this hard of a stance on email, but I do think it should be an option. Here is a link the the posting I wrote about my expereicne with
Zimbra and Spam Arrest.