If the email accounts concerned have not been moved to a VM broadband account then they should have been closed down some years ago. Unfortunately the VM systems have been very poor at implementing their published policy on closing 'orphaned accounts'.
The VM Forum Team will not be able to regain access to the account for you, but they will be able to ensure that they are properly deleted.
They will respond to this thread and will contact you to confirm the account status.
I'm sorry that this will not be the news you were hoping for.
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Then, unfortunately, the mailbox and all the mail within it are gone. The virgin.net addresses are somewhat complicated by the sale back in 2015 (I think) and there is a great deal of blatant BS floating about regarding this, but the basic principle is the same. To make use of a Virgin Media email email address, you must have an active VM broadband account with them, there is no other way of keeping it.
In theory, 90 days after you leave, the mailbox is supposed to be deleted but quite often this doesn’t happen and it lingers on in a sort of zombie state. But eventually it gets tracked down and deleted, which is almost certainly what has happened to your wife’s.