You have come to the right place, Leblob. Are you sure the email came FROM your old email address or was it spoofed somehow? Is your former "closed" email address showing as valid in mailtester.com?
As you know, the mailbox should have been closed AND deleted automatically 90 days after you finished being a customer (according to VM's own Terms and Conditions). But the process fails all too often and leaves the former customer vulnerable to hacking and theft of personal and financial details (IMO, someone should definitely claim for damages from VM or complain to ICO!)
Only VM staff can raise a ticket to get the mailbox deleted manually. The friendly Forum Team will visit this thread in the next few hours (or days?) and then take your personal details by private message.
When they say it is done, you can verify it is gone by running the email address(es) through mailtester.com.
In the meantime, if you still have access via an email client (Try Thunderbird - using IMAP process which synchronises with the mailbox stored on the VM server), delete everything from all the folders to reduce the information available to hackers.
interested to read about old virgin account compromised, that is concerning.I myself now get spam to my primary ntlworld email from both virginmedia and ntlworld address's. I usally check the verify my email services to see if the sender email is a valid email address. I have posted myself about spam email sourced from virginmedia which is happening the last few weeks even with, in some cases valid email address's or some spoofed but they are coming from ip address's which appear to be from virgin servers in the uk. The most recent is a Tesco card scam from virginmediabusiness.co.uk I received this morning from an invalid virgin email, but they are not been detected as spam.
It would appear that some old address's can not be removed as I tried many time to close my primary account or replace it but it is not technically possible to remove on a live account primary email, but should be removable on a closed account or after 150days of inactivity. I am not sure if old address's are recycled to others once an account is closed which is a common practice with most companies, perhaps another person is using your old email now needs checking by virgin.