Hi Antony,
The error you are reporting is issued by a Symantec Mail Gateway product, for which this forum is not for. You may wish to post there to obtain assistance if you have not already done so.
https://www.symantec.com/connect/security/forums/messaging-gateway
With that being said, this errors means the recipient is using a blocklist of IPs provided by Symantec. You can request an investigation with our threat analysts and have them provide feedback to reach a resolution. For more infomation on how to do this, please review the article below.
https://support.symantec.com/en_US/article.TECH82881.html
Regards,
Ben Beaulieu
Sr Technical Support Engineer