You could in theory (but this is like putting plasters on a colander) relocate some of the MIME meta data (Subject:, To:, From:) to the email body and then encrypt it.
So basically obfuscate the MIME headers and use some kind of guid@domain type addresses for the MTA routing.
So basically obfuscate the MIME headers and use some kind of guid@domain type addresses for the MTA routing.