I don't think Google delivers over TLS (it doesn't to my server anyway), so in general your mail is subject to wiretapping anyway. And even if it did there's no way protocol I know (maybe DKIM has a section for destination verification?) for them to know that they're really delivering it to the right host.
Basically unencrypted email is a lost cause already. If you care about this stuff you need to dump webmail right now and go with a client encryption solution (and convince all your friends to use it).
That's good to know. Maybe I have my postfix misconfigured, I'll check. Still, the MitM hole is present. Without destination host verification, the FBI could simply sit outbound on the network insert themselves into the transaction.
Something else is weird; I see both trusted and untrusted outgoing SMTP TLS connections in the logfiles, including untrusted connections to machines that I know have valid startssl certs. Maybe I have my postfix misconfigured as well!
That's between your browser and their server. I'm referring to the content of the SMTP connection over which the mail travels, which remains almost always unencrypted in the modern world. Which essentially means that the FBI doesn't need Google's assistance to wiretap your email per se, they just use and machine they probably have sitting on the backbone pipe anyway.
And again, that's between the client and the server. The path between Gmail's mail server and the sender/recipient of the mail's server is not always encrypted.
Basically unencrypted email is a lost cause already. If you care about this stuff you need to dump webmail right now and go with a client encryption solution (and convince all your friends to use it).