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4yz Temporary Rejections is part of the SMTP Protocol

2011-10-27 16:14:58

Dave CROCKER wrote:

Greylisting is far more obviously a protocol with testable interoperability than is, say, RFC 5234 or 5322.

Pete stated:
[To clarify: I am not committed to greylisting being a protocol. I'm just saying it looks more like a protocol than do 5234 or 5322. It is the claim that there is no way to test interoperability that I reject..]

5234 and 5322 define things that need direct processing by whoever receives the data. There is no intermediary. The interaction between data generation and data parsing/processing is exactly that, an interaction. In addition, some of the data with 5322 specify actual, honest to goodness protocol behaviors. Reply-to directs the construction of a portion of a new email message, for example.

The effect of the proposed capability, in contrast, is entirely indirect. It does not define a new format or a new protocol command or a new protocol response. Rather, it uses /existing/ ones in a particular way. As such, sorry, but those other specs are "more like a protocol" than this one is.

I view the idea of SMTP server's intentional usage of Temporary Rejection with protocol expectation and conformity of a SMTP client Retry standard to use 4yz response, a method, procedure and/or a protocol concept in SMTP.

While the term "GreyListing" is probably poor and dislike by the SMTP purist, replacing it with

       "Intentional Temporary Rejections with a Retry Expectation"

which is part of the SMTP protocol, inherently makes the idea we call Greylisting as part of the "protocol" as well.

In other words, the protocol is the idea that a 4yz response is 100% designed to drive and allow a client to retry the same transactional information at some later point in time. A 5yz is designed to drive the client to not retry again the same transactional information - ever.

What is not part of the SMTP protocol, but part of the GreyList Protocol is the concept of "blocking time."

One may suggest SMTP did document the basic idea of greylisting by using the term "temporary" for a 4yz reply code. It just keep it open ended and did not get into ideas of what is "temporary" and/or how it relates to "time."

That is what this SMTP Extensions hopes to offer - by virtue of an Extension, to optionally add a time factor to the existing 4yz temporary rejection protocol concept in SMTP.

At best, the reason we call "Greylisting" for a temporary rejection falling with a intentional delay may not be part of the SMTP protocol, but that does not make it any less a protocol extended idea to the current SMTP framework.

The difference is random MTA retrying vs a client/server negotiated controlled one - thats a protocol.


Hector Santos