Common misconception. The three letter agencies do not really need to know the contents of your email body. They're much more interested in to/from, timestamps, and subject. Establishing that you communicate with a person and then getting their emails is much easier than playing with your encrypted email body.

I think this is a common misconception of its own. The three letter agencies would love to be able to see the content of messages. But the code makers have run so far ahead of the code breakers that this is effectively impossible. So they settle for only meta information and tell the people that are funding them that this is now sufficient for them to continue to do their job.

What is this based on? How do you know their capabilities?

Per published reports, they (and others) have exploits for many things, including many cryptography implementations.

Exactly right. The American government considers associations inferred from this "metadata" to be sufficient evidence to execute people via drone.

Only foreigners and Americans on foreign soil

This is just such an obviously ridiculous statement.

Feel free to elaborate. This is fairly common knowledge if you Google parts of what makes ProtonMail (and others) susceptible to state actors.

You have a misconception yourself, and it shows. It depends on what they're after. If they want to see an individual's email then obviously they need to decrypt the body.

Base64 is an encoding, not encryption. But yes PGP or S/MIME encrypted email would work.

Charitably, they mean take the whole message with headers, encrypt it, and base64 it so you can stick it in a body. Probably still a bad idea.

This could work on top of GMail, with the help of a browser plugin.