If he's receiving random emails they are probably not important or sensitive enough to be valuable to the government. If he's receiving non-random emails that contain sensitive data, it should all be encrypted.

If you do get something important that is not encrypted, you can forward the mail to yourself as ciphertext.

You're correct that Appelbaum has no control over Google's retention procedures.

The government did not request the contents of the mails in any case, but the header information: who Jake was communicating with, and when.