Re: [ietf-smtp] [Emailcore] Proposed ESMTP keyword RCPTLIMIT

[John C Klensin <[e-mail]>]

--On Monday, March 15, 2021 08:58 -0700 Ned Freed
<[e-mail]> wrote:

>>       SMTP servers have always been able to announce a limit,
>>       in a reply, which means that the client first needed to
>> issue a command.  The mechanism specified here avoids the
>> overhead of that interactions, by announcing limits prior to
>> any substantive interaction.
> 
> Nice. Added.

I wonder.  Along with the "first digit" rule, SMTP has been
fairly clear that SMTP clients should not depend on trying to
parse or interpret reply text except for VRFY and EXPN for which
syntax is actually given for replies.  Assuming this is the sort
of announcement you and Dave have in mind, suppose the client
sends
   MAIL FROM:<[e-mail]>
and the server responds
   250 OK.  No more than 20 recipients
would we seriously expect the client to interpret and use that
information?

In addition, if the answer were "yes", what would prevent a
connection-opening response of 
  220 SNAFU SMTP Server, v 666, no more than 25 recipients
allowed

which is no less an announcement and of course avoids having to
wait for any commands to be issued, even EHLO.

     john