The message is as it is described in the help page http://www.symanteccloud.com/troubleshooting.
The emails are being filtered out by the Symantec Cloud signaturing system. Since the Symantec Cloud service is client managed, the owner of that cloud account can add the sending domain pubseccoms.co.uk to their approved list to bypass the AntiSpam service for the time being. I would strongly recommend to contact Cloud Support directly in order to open a case and start an investigation. This will allow the review of the signature and correct any false positive, or confirm if the block is legitimate.
Cloud Support Team Contact:
US/Canada: +1 (866) 807 6047
EMEA: +44 (0) 1452 62 7766
Australia: 1 (800) 088099
Hong Kong: 1 (800) 901220
Asia Pacific: +852 6902 1130
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Portal: https://clients.messagelabs.com