[ietf-smtp] SMTP Retrying/Sending Strategy on 452 / 4.5.3

[Дилян Палаузов <[e-mail]>]

Hello,

RFC3463 Enhanced Mail System Status Codes defines in 3.6 Mail Delivery Protocol Status:

  X.5.3   Too many recipients

     More recipients were specified for the message than could have
     been delivered by the protocol.  This error should normally
     result in the segmentation of the message into two, the
     remainder of the recipients to be delivered on a subsequent
     delivery attempt.  It is included in this list in the event
     that such segmentation is not possible.


RFC5321 Simple Mail Transfer Protocol says:

4.2.3.  Reply Codes in Numeric Order

  452  Requested action not taken: insufficient system storage


4.5.3.1.8.  Recipients Buffer

   The minimum total number of recipients that MUST be buffered is 100
   recipients.  Rejection of messages (for excessive recipients) with
   fewer than 100 RCPT commands is a violation of this specification.
   The general principle that relaying SMTP server MUST NOT, and
   delivery SMTP servers SHOULD NOT, perform validation tests on message
   header fields suggests that messages SHOULD NOT be rejected based on
   the total number of recipients shown in header fields.  A server that
   imposes a limit on the number of recipients MUST behave in an orderly
   fashion, such as rejecting additional addresses over its limit rather
   than silently discarding addresses previously accepted.  A client
   that needs to deliver a message containing over 100 RCPT commands
   SHOULD be prepared to transmit in 100-recipient "chunks" if the
   server declines to accept more than 100 recipients in a single
   message.



4.5.3.1.9.  Treatment When Limits Exceeded

  452 Too many recipients (see below)


4.5.4.  Retry Strategies
4.5.4.1.  Sending Strategy

  contains: … the retry interval SHOULD be at least 30 minutes…
  and says nothing about 452 or 4.5.3


Is the number 100 randomly choosen?  Obviously, a site can accept 120 RCPTs per iteration, and for the 121st RCPT reply
with “452 4.5.3 Retry immediately after DATA”.

Are clients supposed to retry immediately after receiving 452 4.5.3?

What is the difference between „452” and „451 4.5.3”?  
https://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml says 4.5.3 can be combined
only with 451.

Does „452 Requested action not taken: insufficient system storage” mean in 30 minutes there might be more storage, as
opposed to 4.5.3?

What will happen, if a site has one (1) as a minimum total number of recipients that are buffered?

What is the sending strategy after the total number of recipient per MAIL FROM:<..>... DATA . is exceeded?

How are 452 and 451 4.5.3 supposed to be different than the retry=0 hint?

Regards
  Дилян