I’ve been discussing my ideas on the Lemonade mailing list.
Unsurprisingly, the gurus there didn’t like my attempt to rehabilitate smtps, mainly for political reasons. The RCPTHDR extension probably only has a future as input to the RFC 2822 or RFC 2476 updates.
People seemed fairly keen on QUICKSTART. I have updated it to shave off a couple of round trips from connections that use STARTTLS.
However, a closer look at the CHUNKING spec revealed that it has a very desirable property: it completely eliminates synchronization points once you have got the connection going and you are sending messages. With basic ESMTP, there is a synchronization point at the DATA command, which has the effect of limiting your sending speed to at most one message per round-trip. CHUNKING allows you to send at the maximum speed TCP can sustain. Nice. The CHUNKING RFC needs a little clarification about its interaction with PIPELINING, but its author has a draft update in the works which will do the trick.
I have more to say about sending email efficiently in future essays on how not to design MTAs.