Even better would be a system such as:
You generate a key pair, give Apple the public key. You manage your own private key.
Then, for each email:
Apple receives the email as plain text from another server (likely via SSL), encrypts it with your public key and stores it on their servers. When you connect to retrieve your mail they send you the encrypted blob that you decrypt via your private key.
Problems are this: first, Apple has a plain text copy of each email you receive and could be asked (nicely or forcefully) to record this somewhere before proceeding with encryption and storage. Second, replies are not encrypted.
Much better for users to manage their own keys and use S/MIME. That's what I do with my friends and Google (my email provider) never sees unencrypted messages from me to any of my friends and vice versa.