And, even if you use a side-channel to distribute keys between the sender and receiver (to encrypt the data safely), the header absolutely has to be plain-text.

And, even if you use a side-channel to distribute keys between the sender and receiver (to encrypt the data safely), the header absolutely has to be plain-text. Governments and companies scraping email meta-data is already a huge problem. A protocol closer to Tor would make for much more secure email distribution, but it would also require a complete protocol rewrite. Potential death of email from a back-end point of view at least.