oops Yes i had the virgin.net account when on landline, then I upgraded to cable bb and got a new virginmedia a email address. I use both of them for different things. Periodicaly I go into them via the web site to clear all the old rubbish out. Tried to do it today and could not access the virgin.net.
Also I am going to guess that you are trying to access via webmail. Webmail access is the first thing to close, although the email mailbox may still exist in virgin's servers, hence it shows as valid in mailtester. You can probably continue to access the address using an email client, but the mailbox could be deleted at any time, or be hacked by any one who can guess the password. Then your personal and financial details could be stolen and your friends and family spammed.
Please log into your billing account and see if the address is showing in the my profile or manage accounts tabs (see photos below)
If not showing in your billing account and you want to get permanent access to it, you can possibly ask VM to perform a Move & Transfer (not guaranteed to be available). But this will delete any existing email addresses you have - the email addresses are known as a profile and have to stick together. This is for data security purposes (IMO).
So the best thing may be for you to back up any thing you have in the virgin.net email system, get working on transferring anything you need to other email addresses (preferably non-ISP and better security so you don't have to do this again if you move ISP). THEN ask VM to permanently delete the mailbox to save you the agony of being hacked.
Merry Christmas to you as well. It is best to know the worst now and have time to deal with it, rather than find it is suddenly gone and you can't update your amazon account or whatever. It happens to loads of people who come to this forum every week.