@Sa5 according to VM’s terms and condition, the email address and all the email contained within it absolutely WILL be deleted 90 days after you leave. So far so good, in practice, there are many posts here reporting that email mailboxes and addresses have continued to work of months, if not years afterwards and suddenly been deleted with, apparently, no notice. Sorry, no you did have notice, it was in the Ts and Cs, you just don’t follow them.
Now to be honest VM do themselves no favours at all by not promptly killing off old email mailboxes and addresses after the 90 day timeline, but it is what it is. Personally I would work on the basis that it will die after 90 days; but might not, so make sure that any contact / third parties service has been told about your new address and is using it, Then when you leave, just delete any mention of the old VM mailbox from all of your devices and assume that it no longer exists.
Now if VM fail to delete it by the deadline, then is this an issue? It’s just some random email address which isn’t tied to any services of yours’ who who cares if it is compromised!
Oh you did delete all of the email within it before you left didn’t you?