Sounds like US citizens should have an unencrypted email signature saying:
My name is XXX, I am a US citizen.
My address is YYY.
By the rights guaranteed by the US Constitution and supported by federal courts, the contents of this email are encrypted, private, and require a valid warrant to be intercepted, stored, or accessed by the US government.
My name is XXX, I am a US citizen. My address is YYY. By the rights guaranteed by the US Constitution and supported by federal courts, the contents of this email are encrypted, private, and require a valid warrant to be intercepted, stored, or accessed by the US government.