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2011-10-12 07:29:28

At 11:16 -0700 on 10/11/2011, Murray S. Kucherawy wrote about Re: We need an IETF BCP for GREY LISTING:

RFC3339 instead of ISO8601, perhaps?

Of course, abusers will only pay attention to this if it benefits them and it¹s cheap to do so. But they won¹t be distinguishable from legitimate clients that just don¹t know about this extension and retry by their own schedules, so one can¹t penalize such clients for not respecting the request. On the other hand, you might be able to identify ³good² clients (for some value thereof) by observing which ones do respect the request.

The way I understand the suggestion, the MTA offers the sender (in its 220 reply) the information that it can send such a reply and the sender acknowledges that it has support to respond to the 4xx reply. Absence of such an acknowledgement would suppress the sending of a 4xx "retry at" message (you would get a different type of 4xx).

On Oct 11, 2011, at 1:45 PM, Steve Atkins wrote:

In that case, some minor extension to allow the SMTP server to communicate something a bit more nuanced than "Go away, come back later." might have some value.

I could see value in that.  I could imagine an SMTP extension which, if included, indicated that the server might send a response of the form

4xx please retry between <date-time>-<date-time> response to say the MAIL (or maybe DATA) command, where <date-time> could be an ISO8601 date-time (the horror!) based on GMT (Z) and with no punctuation.  If the client included a SIZE the server could even do bandwidth reservation.  Of course there would be no guarantee that the second attempt wouldn't result in some sort of 4xx response for other reasons.

Greylisting servers could certainly make use of it, though I don't know if it would be a good idea to recommend that greylisting servers use it.