We are having the same issue as a ticket below and their summary perfectly summarises our issue:
I have been struggling to get in contact with someone to get some old routing for a domain no longer protected by MessageLabs removed. Would any be able to point me in the right direction for getting assistance with the following as I'm unable to raise a support case without a site ID and the domain's owner was never a direct customer of Messagelabs.
The domain used to be hosted by a provider who used Messagelabs/Symantec Cloud to protect all their customers. They however cancelled their contract over a year ago and moved things in house without removing the routing on the MessageLabs system.
We didn't realise this was an issue until the domain was migrated to a new e-mail provider and messages sent by MessageLabs/Symantec Cloud customers were no longer being delivered as they were not using the updated MX records pointing to the new provider.
Can I please have a contact to get this removed? We can prove ownership of the domain. Thank you