I'm using Zimbra 4.5.7 and recently started having an issue with being blacklisted through spamhaus.

My ISP informs me that something is incorrect in the header

for example, in the header is this:

Received: from host.domain.com (unknown [IP Address])

They are telling me that we are being blacklisted due to the "unknown"

Our mail server is not behind a NAT, it has a dedicated static IP and the $myhostname is setup correctly, along with the $mydestination/$myorigin

Any ideas?