[baudehlo]

It also misses one of the biggest fundamental problems with SMTP: Lack of per-recipient replies. Though you could potentially mandate a single recipient per request. We wrote a plugin to emulate that in Haraka, where you just 400 (temp fail) each subsequent recipient. The downside is some sources just plain don't retry.

[inigyou]

LMTP is a small modification of SMTP with a per-recipient response code - but I'm not sure what it's intended for. It isn't used on the internet. It's sometimes used as IPC within a mail server.

[baudehlo]

LMTP is for delivering to an IMAP (or JMAP) mailbox, generally. We also have a plugin for that. But it has certain flaws for exposing it to the internet (I just forget what they are right now).

[inigyou]

There's no such thing as an IMAP mailbox. Mail receiving is split down the middle. Your SMTP server stores the email in a folder and your IMAP server provides access to that folder.