To be a little blunt, email was NEVER designed to be secure from its invention and if you're using email to transmit sensitive information you have to encrypt it before it even leaves your machine. I personally set up OpenPGP using GNU Privacy guard (and there's a Gmail plugin that once you set up is super easy to use), unfortunately I'm the only person I know that uses it, so I can't send any protected email.
I don't have any real privileged communications right now, but if I ever need a lawyer I'm either going to insist on OpenPGP or tell them I simply can't email them. If I were to hire a PI such as yourself, I probably would insist on the same thing. Even if they pass a law that doesn't let the government look at email without a warrant, that still doesn't change the fact that email is inherently insecure, and if you're sending anything unencrypted (I'm talking contents encrypted, not SSL) it's in the open anyway.