If you'd told me a year ago that email was dead, I wouldn't have believed you. This year, I just might believe it. My mailbox is perpetually full of notifications from other services and most of my actual work has moved onto various managed systems ...
Anyway, so here's one from the Ideas I'm Never Going To Implement Myself bucket.
NoMail is an email service which doesn't store anything. Anything at all. Email received by NoMail is ephemeral and exists only in your client. While you don't have the client open, email sent to your NoMail address bounces.
NoMail may be useful for a few different things: password recovery, signing up to overly intrusive services, avoiding contact from humans. Any sense of security it brings is totally dependant on the server's willingness to forget things, so it probably isn't a good way to avoid Echelon.
NoMail doesn't have usernames or passwords. It doesn't even remember which email addresses exist.
Instead, the user just enters in a pass phrase, and NoMail uses a KDF to calculate a key. The key is then hashed and used to derive an anonymous email address using one of the umpteen pronouncable password generation algorithms.
The key is used by the client to connect to the server and open a WebSocket connection over which it can receive email.
When an incoming email arrives at the NoMail server, it first checks if a matching user is currently connected. It can do this by keeping an in-memory lookup table of the derived emails of all logged in users.
If the address is not found, it refuses the email. Generally the mail will then get spooled on the sending server and retried a few times before giving up and being bounced back.
But if a matching client is connected, the NoMail server will accept the email, and shuttle it straight through to the client on their Websocket in classic "mbox" format: headers, attachments and all. The client can then parse the message for display.
(Alternatively, attachments could be removed and stored somewhere briefly for retrieval over HTTP ... but that seems to defeat the purpose somewhat)
The NoMail server can also relay outbound email with DKIM signature etc, using the derived key. The address can be calculcated from the key, but the key can't easily be calculated from the address, so knowledge of the key proves ownership of the email address.