Yes, the email address mailbox should be deleted 90 days after your broadband account closes. But daily anecdotal evidence here on the community forum is that the address is only partially closed, leaving the previous customer vulnerable to hacking, theft of personal and financial details, and spam being sent to family and friends. So when you are 90 days gone, run the address(es) through mailtester.com and verify they are completely invalid. If still valid, come back to this community for assistance from the VM Forum Team who can get it manually deleted.
"Can I keep my Virgin Media Mail service when I disconnect from Virgin Broadband?
If you disconnect your Virgin Broadband service we provide access to your Virgin Media Mail account for 90 days after your service closes. This should provide sufficient time for you to save any contacts, emails and files you wish to keep. After this time your Virgin Media Mail account will be closed."