If you want it protected, don't leave it in clear text in the custody of someone else who may or may not choose to hand it over to the authorities without your permission.
In other news: you can't invoke the 4th amendment if you stash boxes of weed at a friend's house and he hands them over to the government when asked.
You are consistently morphing the issue and providing bad analogies.
There are privacy policies and terms of service that cover what a service provider will share with whom and under what conditions. This represents an agreement between the user and the service.
However, that is wholly different from the rights held by the government (IRS) to compel the service provider to provide information at the government's demand.
Your analogy with the "boxes of weed" are similarly misguided and completely unrelated. The more relevant analogy would be that the government forced your friend's landlord to unlock his door without a warrant, entered, discovered the boxes of weed with your name on it, then arrested you.
And, to put your analogy back into the subject e-mail context, if you e-mailed something incriminating to your friend and your friend opted to turn you in to the authorities, that is entirely different from the government gaining access to your e-mail of its own volition, and without a warrant.
So if I send a letter via USPS do I have the right to expect that someone will not intercept and open that letter without getting the correct permission?
Mail theft was a pretty serious crime in USA I am led to believe, regardless of whether it is sent in encrypted form or not...
In other news: you can't invoke the 4th amendment if you stash boxes of weed at a friend's house and he hands them over to the government when asked.