It is from many months that an our IP is inserted in your blacklist.
When you try to use your IP removal tools we obtain the following response:
The IP Address 188.8.131.52 was found to have a negative reputation. Reasons for this assessment include:
We ask to investigate, after we checked all configuration of our server.
If we ask to other sites as Sender Score o Cisco Talos the IP is marked as Good Reputation.
No other blacklist reveal our IP as listed.
In this moment, your blacklist, blocked many our clean mail.
We ask to remove our IP from your list or indicate clearly what type of problem we must resolve.
All I can tell you is that you will need to contact Symantec Support to assist you further.
In researching this issue I have also found the below items:
Symantec Support "IP Blacklist Removal"
Check IP Reputation
IP Address Investigation