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Re: [ietf-smtp] Per-Recipient Data Responses

2014-03-05 18:28:06
Brandon Long <blong(_at_)google(_dot_)com> wrote:
For us, the most obvious use case is for enterprise routing rules.
For example, you can imagine that a school might implement an
objectionable content filter for students, but not for faculty.  In
that case, a message containing such content that was in the same smtp
transaction could now be allowed to the teacher and rejected by the
student.  Today, we have to accept for both and then generate a
Anyways, the obvious use case is per-user filters (spam or rule
based), neither of which are really that strong, I'd think.

At my two most recent employers, we referred to this situation as a "disposition conflict": one Email message, but different policies configured by different recipients created a situation where it wasn't possible to convey a single response (250, 450, or 550) to the sender. There are many more ways to get in this state than just spam filters: policy rules that allow message bifurcation, for example, or data loss protection filters that defer messages for administrative action or kick them back to the sender.

While store-and-forward MTAs have ways to resolve this, SMTP Proxies do not: there's no place to queue messages for retries, and no ability to send DSNs. So instead you implement a long set of rules ("if this, then this") to handle the conflicts in the least surprising way; but the result is still unsatisfying and occasionally quite surprising.

If you're responsible for an SMTP proxy, PRDR gets much more interesting.


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