On 21 Feb 2013, at 17:13, Chris Knadle <Chris.Knadle@???> wrote:
> On Thursday, February 21, 2013 10:30:10, Todd Lyons wro
>
> I pulled the Exim master and found reference to the expired draft PRDR spec in
> /doc/doc-txt/experimental-spec.txt:
>
> http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt
>
> PRDR requires opt-in by the sending MTA during the MAIL FROM command, and also
> changes the response after the DATA command if there are per-recipient
> handling differences. This means that it only works with other MTAs that also
> implment PRDR, which at the moment sounds very rare.
Yes, that's right. But that's the only way of doing it. In effect, the two MTAs are agreeing to speak LMTP (I'm assuming the response list looks the same?) instead of SMTP. And that's because SMTP doesn't have any way of giving per recipient responses after seeing the data.
> If the other MTAs implement this it could be very worthwhile, but I'm not sure
> what the chances are of the MTAs getting support for this.
Well, I think the chances are quite good. It's clearly desirable for recipients to implement this, and given that it's entirely optional, I don't see any reason to have this off by default in Exim - except initially while confidence is gained. Exim has a substantial market share (see
http://www.securityspace.com/s_survey/data/man.201201/mxsurvey.html for example).
It's also worth noting that a substantial proportion of mail, judging by my logs, has only a single recipient anyway. I can imagine that bulk senders might want to use this for efficiency reasons, since it will mean that they have a chance. Bulk senders might prefer PRDR to VERP, for efficiency reasons. They could, for example, decide to send to multiple recipients when they encounter PRDR servers, but use VERP otherwise - it's a decision that could be made on the fly. In fact, that would be an interesting feature to offer: Exim could generate VERPed bounce messages (for a local list server) in response to PRDR rejects.
I can forsee recipients giving a small positive reputational boost to senders who implement PRDR.
> -- Chris
>
> --
> Chris Knadle
> Chris.Knadle@???
>
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Ian Eiloart
Postmaster, University of Sussex
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