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

2019-02-16 11:33:36

The SMTP-GREY extension fits with RFC5321, Section 4.5.4 "Retry Strategies."

Note the section sending strategy MUST requirement for a delay and added considerations for more advanced strategies that can shorten the 30 minutes SHOULD:

   The sender MUST delay retrying a particular destination after one
   attempt has failed.  In general, the retry interval SHOULD be at
   least 30 minutes; however, more sophisticated and variable strategies
   will be beneficial when the SMTP client can determine the reason for

That is what the SMTP-GREY "retry hint" was intended to do, not get into a more complex client/server negotiated retry system where all sorts of factors are considered. The compliant SMTP client MUST delay the retry and the extension fits with the extended "sophisticated and variable strategies" consideration described above to adjust the retry interval.

Beyond this, IMO, it would be a different document like a BCP or Information status, perhaps titled:

        "SMTP Outbound Mail Optimization Strategies"

that will serve bulk mail senders in today's environment. It would apply in general for remote servers who have deployed greylisting methods for load and/or spam controls.

As far as SMTP-GREY, maybe a (draft) clarification for the time-value can be appended to section 2.5 "Recommended Blocking Times:"


   Using a time-value of zero for the reply hint may be used by
   the Greylisting Server in an attempt to convey to the compliant
   SMTP client, the server has no blocking time and the client may
   retry without delay or follow its normal retry schedule.

   Since a non-compliant SMTP client will always follow a normal
   retry schedule, it is recommended the Greylisting Server who wishes
   to reject the first attempt with little to no blocking time for the
   2nd attempt to consider using a retry hint of one second for backward
compatibility reasons. Using one second should effectively produce the
   same almost immediate retry the Greylisting server maybe looking for,
while using zero seconds may not produce the desired effect of an immediate client retry and cause an undesired delay, i.e. the client could default to
   30 minutes as suggested by RFC531.


On 2/16/2019 9:25 AM, Дилян Палаузов wrote:
Hello Santos,

your draft describes an existing server’s defering strategy and advices clients 
how to act, when this defering strategy
is in effect.  One of the essential parts of your draft is writing down the 
idea (formalizing) for the smtp-clients to
reuse the IP address on retrying, when grey listing is in effect.  Whether grey 
listing is in effect, does not depend on
a 250-GREYLIST or retry= information.

I suggest you include in the draft how a client can recognize that greylisting 
deferring is in effect and, based on the
outcome, decide to use or not other IP addresses for subsequent/immediate 

If retry=0 is (intentionaly) left undefined, then the draft shall state this, 
and probably suggest what clients shall do
on retry=0.  (E.g. ignore).  Keeping retry=0 this way, because anything else is 
not backward compatible with
implementations of a draft, does not necessary mean that there is a consensus.

Your draft states:
    If a SMTP server offers a "retry=time-delay" hint which results in a
    wasted 2nd attempt and requires additional attempts, the SMTP client
    MAY begin to ignore the server's "retry=time-delay" hint after the
    2nd wasted retry.  The SMTP client implementation can decide what
    limits to place on honoring "retry=time-delay" hints and wasted
    attempts it provides.

I suggested previously, and this was not tackled, that a retry is not wasted, 
if the remaining number of recipients has
decreased during the retry.


On Wed, 2019-02-13 at 15:27 -0500, John Levine wrote:
In article 
<12803(_dot_)1550087591(_at_)turing-police(_dot_)cc(_dot_)vt(_dot_)edu> you 
On Wed, 13 Feb 2019 12:22:09 +0000, Дилян Палаузов said:

Is publishing 1024 distinct IPv6 addresses for MX on a domain a good idea

Only if you're *really* sure that everybody who wants to talk to you supports
at least EDNS0 and doesn't block tcp/53 :)

Let me simplify that answer:

No, it is not.


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