@bencrozier actually, no it does make sense. Firstly are you a current VM broadband customer? If not or if you are but have had a break in service, then there is a good chance that the email address has been deleted or is in the process of deletion.
As regard to using it as a login for another account, that'll still work - when you originally signed up to this service it would have wanted a username and password, email addresses are 'globally unique' so they are convenient as usernames as nobody else in the world can have the same address. You sign up, the email address is checked at that time (usually by sending a verification message) but from then on it doesn't matter if the email address is 'live' or not - it's just regarded as a username - it's never checked again. Unless, of course, you need to change something, when they might want to send a verification message to a, now, nonfunctioning address.