[Emailcore] Proposed ESMTP keyword RCPTLIMIT

[Jeremy Harris <[e-mail]>]

On 11/03/2021 23:29, Viktor Dukhovni wrote:
> <quote 3>
>     When a conforming SMTP server encounters this condition, it has at
>     least 100 successful RCPT commands in its recipients buffer.  If the
>     server is able to accept the message, then at least these 100
>     addresses will be removed from the SMTP client's queue.
> </quote 3>
> 
> I wish that were true.  It would certainly aid interoperability.
> But the said 800lb gorilla providers have felt free to dictate
> arbitrary limits below the RFC requirements

This is outside the current work item, but:

We should consider a new method for the server to advertise limits
on RCPT before use; this would be more efficient given pipelining.

Perhaps an ESTMP keyword "RCPTLIMIT" taking parameters
   MAX=nnn
   SINGLE_DOMAIN

These would be advisory for the client; the server would still have
to apply limits via responses to RCPT commands, and the client MUST
NOT take any different action to command responses based on the EHLO
keyword seen.  The client SHOULD choose command sequences based
on the EHLO keyword parameters, for example by issuing no more than
<nnn> RCPT commands in any single transaction performed on this connection.
-- 
Cheers,
   Jeremy