Hi. I thought the same - I use zimbra primarily as a mail gateway to an exchange box. Emails from blocked domains were still hitting the exchange box and bouncing back to zimbra - the problem was the format of the sender_access list - it had been copied from an existing list used on the exchange box - it looked ok but where 2 domains were listed, there was @ before the domain and one had *@domain.com listed. Removing these and remaking sender_access.db fixed the problem and all mail was rejected correctly by Postfix rather than hitting the exchange box.
I'm using ZImbra 7.1.3 OSE on Ubuntu 10.04 - check your sender_access file.
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