[Asrg] where the message originated (was: DKIM role?) (SM)

[der Mouse]

> a) when a message has two or more local recipients, you can't have
> different policies for each recipient about what to reject.

Sure you can.  You just need to push that information to your border so
you can 4xx all recipients whose rejection policies differ sufficiently
from those of the first 2xxed recipient.  (Where differing
"sufficiently" means differing in ways such that the SMTP body contents
could cause them to make different rejection decisions.)

> There are some workarounds, but they're kludgy and cause their own
> problems.

I implemented the above, once.  The only problem I saw was when
accepting mail from broken MUAs that tried to send not with SMTP but
with some ill-defined SMTPish protocol in which a 4xxed recipient meant
the mail had failed entirely.

Even if you have to accommodate such brokenware, all this means is that
you mustn't do differential recipient rejection for MUA submissions.
Unless you know of an MTA that misbehaves that badly?

> b) when you reject a message you can try to explain why, but often
> the sending MTA will throw away some or all of your explanation.

And that's your problem...why?  There are lots of ways that information
could get lost.  The end user could ignore the bounce, too, or may not
be competent to read the language you generate it in.

> If you generate a bounce, you have full control of what the bounce
> recipient sees.  For example, you could include an explanation of why
> they should lobby their domain controller to publish SPF records.

And you really think that anyone who doesn't already grok bounces is
going to have any clue what you're going on about?

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