The e-mail account gets cancelled soon after the associated broadband account is cancelled.
You could purchase a single POP3 mailbox from us, and ask for it to be set up with the same email address as your previous Zen one. This could then be cancelled also once you were no longer receiving e-mail to it. This product doesn't appear on the website, but you may still be able to purchase it by calling sales (IIRC its about £12.50 /year)
You could always ask customer services if they could set up a temporary forwarder for you, but given that you are no longer a customer I'm not sure if they'd be able to do that or not, but it doesn't hurt to ask
Kindest regards,
James Sweet
http://www.zen.co.uk