Leave the priority for this incident (delete others).
the RBL cbl.abuseat.org was getting returned the Open Resolver IP DNS response (127.255.255.254) when checking the IPs against one of our US-based DNS servers (220.127.116.11).
On all our TitanHQ-hosted DNS servers we have configure our DNS servers to discard all DNS request to cbl.abuseat.org, which should stop this these from happening.
The RBL cbl.abuseat.org was part of zen.spamhaus.org, so their is no reason to have cbl.abuseat.org in your list of RBL servers. If cbl.abuseat.org is in your list, you should delete it, since now it will not be responded to.
Also, you should -Flush- the DNS cache in System Setup > Network > DNS Settings
This would have causes all a very high percentage of all mail to be dropped as an RBL Block if you have cbl.abuseat.org in your list of RBL Servers in System Setup > Mail Relay > IP Controls > RBL Servers
On our DNS servers, those requests will no longer be responded to, so incorrect RBL rejects related to cbl.abuseat.org will no longer happen.
Note: Even Self-hosted SpamTitan should not use cbl.abuseat.org as a RBL server (since it is include as part of SpamHaus).