GPGOL not re-encrypting emails

I was under the impression that GPG should be re-encrypting sent/received emails. If I receive an encrypted email and I decrypt it to read, I expect the email to stay encrypted as it is stored in my Exchange mailbox. This is not the case, however. The emails that I’m receiving after they are decrypted are staying decrypted in my mailbox. Is there anyway to make it so that the messages are re-encrypted so they can not be read by some nosy Exchange Administrator with access to my mailbox??