I'm guessing that in order to use the email service for your email address, you keep on having to pay. If you had the .mac email address before they switched to MobileMe, then you kept your previous email address.
All that really matters is if your email address is valid at the time of registration to make sure you say say who you are. After that, all your email address really is, is a simple user name. iTunes doesn't check whether or not it is still valid. All it wants is to find your information, which is listed under the .mac email address that you had.
I believe that once you have stopped paying, you cannot get it your email address back, unless of course you reregistered before they stop taking registrations. Apple said that you could only keep your MobileMe address for iCloud if you did not close your account for MobileMe. Which is mainly the reason why I'm not taking the refund that Apple will give me for closing my MobileMe account now.