JMAP: Like IMAP but Not Really

Alternate but similar - sender holds the email until the receiver fetches: https://cr.yp.to/im2000.html

I'm not sure it actually helps that much to cut down spam but anything is worth a try at this point.

Email is designed around asynchronous clients, e.g. the ability to write everything offline, connect for just long enough to queue them all up in some server, then disconnect while that server passes them along. By the time the receiver (server or client) knows that they've been sent a message, the sender may be long gone.

Hashcash only works if:

- It can't be bypassed. Without a mechanism to tell senders how much to include, receivers must set their price near zero to avoid discarding legitimate messages which guessed the wrong amount. Keeping messages without any/enough hashcash would defeat the purpose of the thing.

- The sending client does the work. Since servers are always online and reachable via a known address, we could have them negotiate an amount; e.g. the receiving server checks the message for hashcash, returns an error stating that a certain amount is required, the sending server tries again with more hashcash. The problem is, getting the sending server to mine hashcash won't stop spammers: they'll just use other people's servers, like gmail, hotmail, etc.

I've written about this before, but I think that a generic protocol for negotiating hashcash would be really worthwhile. Maybe it could be made to work for email, but even if not there are plenty of synchronous protocols which could use it.

In particular, there's no reason to keep a fixed price; we can figure out a price using the same heuristics as existing spam filters: can we verify the sender, have we seen spam/ham from them before, do they appear on black/whitelists, etc. This way the pressure can be kept on spammers, whilst the majority of normal traffic can go through with little effort. Note that this fixes the mailing list problem too, e.g. if users add the list address to their whitelist, or if they send a message in order to subscribe (hence triggering the "allow this, it's a reply to our message" heuristic).

I also think this would be a nice alternative to API keys, since it would keep things more "open" for experimenting and mashups, whilst giving providers a way to avoid abuse (API keys could still be provided, as a way to significantly lower the amount of hashcash required).

"Email spoofing bugs do not qualify. We are quite aware that users can set arbitrary From addresses on emails, that our SPF records allow arbitrary hosts to send email as our domains, and that our DMARC policy is not enforcing passes. These policy decisions are by design, and we track the actual sender in a separate header."

[1] https://www.fastmail.com/about/bugbounty.html

Describing this as REST is really strange. Defining your own operations over an HTTP POST is what SOAP and other RPC style web services do and specifically what REST isn't. But I guess that a lack of a standard behind REST ended up with the term being used for everything.