When you leave Cambridge (student or staff) your Computing Service accounts (except Raven) will be cancelled, as described in the pages on Staff leaving the University and Students leaving the University. This includes your Hermes mail account and your @cam mail address.
Autoreply service for @cam mail
Many staff or students leaving Cambridge have already publicised their @cam address widely outside the University. The Computing Service cannot forward mail sent to your @cam address after you have left, but you can arrange for an auto-reply, giving a contact email address, to be sent to anyone who emails you @cam. This should be set up before you leave Cambridge, and is done via Lookup. If you need to change the address subsequently, you can do so using your Raven password for access.
You may already have another, permanent, email address that you can use; if not you will need to acquire one. If you are an alumnus of the University of Cambridge you can join the cantab.net service, see http://cantab.net/ for details. Some professional bodies offer forwarding email addresses to their members, but you will need to remain a member of that organisation. Alternatively you may wish to use one of the big free mail accounts, for example Google (gmail) or Microsoft (hotmail).
Details of how to enable your change-of-address autoreply are available.
Moving your existing mail folders
You will need to cancel all your mailing list subscriptions - see Cancelling or suspending list membership.