I am running ZCS 4.5.10GA. It works wonderfully! One issue:
All mail from a certain domain is being queued and not delivered. It is sitting in the incoming que. I have added the domain to all the whitelists and the av bypass list. No luck, it still sits there. Here is the actual error:

Sender: jkimbal@problemdomain.com
From Host: outbound.mse79.exchange.com
From domain: problemdomain.com
From IP:
Recipients: bob@mydomain.com;jake@mydomain.com
To domain: mydomain.com
Size: 0
Reason: undefined

Any idea on why all mail from problemdomain.com are being queued?

(Names have been changed to protect the innocent!)
I used this article as a basis to add the domain to the whitelists and the anti-virus bypass:
Improving Anti-spam system - Zimbra :: Wiki