The sentence directly before that, from Mark's original e-mail, which I elided, is now horribly sad given your question and today's context.

"""If you're just curious as to the reasons, and don't wish to argue it, I'll gladly explain in private email (offline from the list)."""

To be clear, that was all a quotation from an e-mail written by Mark Crispin: I do not really claim to be an expert in an of this yet, as there is still much more reading I have been meaning to do regarding Lemonade.

I guess I can try, however, to find more Mark Crispin quotes from various mailing list posts to help answer the question; he seemed heavily involved in the 2003 discussions of the Lemonade Submit proposal.

"""You've given an argument for creating a better submit protocol than SMTP, but not for making IMAP be that submit protocol.""" --
http://www.ietf.org/mail-archive/web/lemonade/current/msg000...

Things seemed to come to a head in 2005 with an argument between Marc Perkel and Mark Crispin. This argument actually resulted in Mark Crispin petitioning for Marc Perkel's removal from the mailing list.

"""> IMAP can move messages to and from the server. So if you have a protocol that can deliver messages to a server then why not let IMAP hand off outgoing email to SMTP?""" -- (Marc Perkel)

"""The fact that two protocols have related functions does not mean that those two protocols should be merged into one protocol. Otherwise, everything would have been TELNET protocol in the 1970s, and HTTP protocol today.""" -- http://www.ietf.org/mail-archive/web/lemonade/current/msg014...

Concurrently to this (possibly causing it) was a new thread started by Mark Crispin with the subject "SMTP-over-IMAP harmful" that has a long list of reasons for why IMAP should not be extended to allow for directly sending mail.

http://www.ietf.org/mail-archive/web/lemonade/current/msg014...

It is probably most appropriate to read that message and that thread to get a summary of the kinds of issues being discussed, so you can get statements "straight from the horse's mouth".

(For clarity to those not familiar with the various e-mail specifications, I will point out that the "submission" protocol mentioned is the port 587 "for users, not e-mail servers" variant of SMTP, not the IMAP Submission specification; though, watch me actually be wrong ;P.)

This isn't the complete context of the discussion, though; that would require spending much more time digging through these mailing lists posts. From the chair of the committee:

"""1. For the folks that have been here from the beginning, please have a little patience when someone comes in with an idea that is 10 years old, got a year of intense scrutiny, and got rejected. Not everyone comes in with the amount of collective wisdom that you have. [Note appropriate use of "you". I mean "you" here.]""" -- (Eric Burger) http://www.ietf.org/mail-archive/web/lemonade/current/msg014...

Mark Crispin actually wrote a memo, called "Message Submission" which provided alternative ways to do this, and which involved a bunch of things I have not looked much into yet (such as BURL). However, this memo also came well before the comment I quoted in the message to which you replied with this question.

http://tools.ietf.org/html/draft-crispin-lemonade-pull-01