I noticed today the same problem today.
We have multiple messages in log with:
554 5.7.1 Service unavailable; Client host [xxx.xxx.xxx.xxx] blocked using zen.spamhaus.org
When I check IP on spamhaus, some IP addresses aren't listed there.
Not sure if this is spamhaus problem or centos webpanel.
Only fast solution was to disable spamhaus in postfix i /etc/postfix/main.cf