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[ietf-smtp] Reviving PRDR

2019-01-10 17:44:06

On Thu, 2019-01-10 at 16:07 -0500, John Levine wrote:
In article 
<13753(_dot_)1547150864(_at_)turing-police(_dot_)cc(_dot_)vt(_dot_)edu> you 
On Thu, 10 Jan 2019 11:55:53 -0800, John Bucy said:

The MSA could call ahead/cut-through, doesn't exim do that? That might 
allow for the mua to throw an error ui at the user if they fat-fingered 
recipient rather than getting a bounce back later.


Consider this reply.  I'm in Comcast cable territory, which means that I can
only do outbound port 25 to Comcast/Xfinity servers.  So my only realistic
way to get this mail out is to 587 it to Google's submission servers.

Now how do I "call ahead" for the cc: that's going to John Levine?

The theory is that you submit the whole message to Google and it
probes the recipients before it accepts the message, but now you have
the added issue of how to report back that receipient A can handle it
but recipient B cannot.

PRDR (per recipient delivery response)?  Just like 
LMTP-after-data-per-recipient acceptance/rejection, but for SMTP,
possibly adding extra DSN codes.  PRDR solves other also problems, like stating 
“the first recipient accepts the
message, and the second recipient does not” (e.g. the second recipient has 
different spam rules).

Last time PRDR was discussed here was 2014.  Since 2013 it is integrated in 
Exim and back in March 2014 it was not
enabled by default.  Today in Exim enable_prdr still defaults to False in the 
documentation, but the
src/configure.default sets it to true.

Why isn’t enable_prdr in Exim enabled nowadays by default?


PRDR specification →

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