I think the end-to-end encryption of email is an important problem, but even more important (IMO) is that email is no longer decentralized by any reasonable standard.

The reason, if I had to guess, is probably primarily because of spam. But these days, with ubiquity of high quality encryption standards in place in most other realms of the web, why not in the validation of email transmission between servers?

Here's an idea. Even if we generally still only have encryption at the protocol level in email, why not incorporate a new header into the email message itself (let's call it signature). Now anyone who deploys their own email server can deploy their public key(s) for email in a TXT record in DNS, and any recipient of email can now (for most practical purposes, without substantial work on the part of the malicious hacker) guarantee that the message was sent by who the sending server says it was sent by.

I think a wide-spread open standard as simple as this could help re-decentralize email while not causing additional fear that spam will again make our lives miserable.