What do you get from running this on the zimbra server:
Code:
dig 40.56.209.64.zen.spamhaus.org
This IP address recently tried to send me spam and was blocked using zen so at least as of 11:05 Oct 27, 2009 PDT it was listed.
And no answer from spamhaus for an IP address simply means it isn't listed... not sure why you would be getting an error telling you that.