The majority of new postgraduates are issued with an account on the Computing Service mail system, Hermes, shortly before they arrive in Cambridge. If you expect to begin your course in October your email address will be available from late August, if you will start in January then the account will be set up in mid December, and if you start your course after Easter your account will be available just before Easter.
You will be able to find out what your email address will be before you arrive at Cambridge. You can forward mail to your Cambridge account once it is open, but you will not be able to read mail until you arrive in Cambridge and collect the password for your account.
If your email address does not appear in the email search, possibly because your position was not clarified until late, then, when you arrive in Cambridge, you should contact your College Computer Officer for advice.
More information for students is available.