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Re: Last Call: <draft-ietf-appsawg-nullmx-05.txt> (A NULL MX Resource Record for Domains that Accept No Mail) to Proposed Standard

2014-07-21 10:15:36
On Sat, Jul 19, 2014 at 10:38:14AM -0400, Dave Crocker wrote:

No, it designates non-/receiving/ domains:

That's a bit of an under-statement, because, for example,
with the widely practived:

    reject_unknown_sender_domain

restriction in Postfix, mail from nullmx domains *is* rejected,
so any that declare-themselves non-receiving become non-sending
in practice.  If the draft suggests that nullmx domains might
expect to send mail will impunity, that would be misleading.

In practical terms, if you don't receive, you can't send (that is
you can't use the domain in the SMTP return path).


Sorry, no.

   1. The return-path is not required to have any relationship to any
another email field.

   2. The semantics of nullmx make an assertion only about receiving.

In fact, there are legitimate scenarios that include use of domains used
for sending but not receiving, as the draft cites.

The fact that some operational choices might not allow this scenario is
a separate matter, outside of IETF specifications.

One can bury one's head in the sand if one likes, but non-receiving
domains that turn on nullmx (or whatever it ends up being called)
need to be aware that in practice they will find their mail rejected
by many receiving systems if they attempt to use such a domain in
the "MAIL FROM:<return-path>".

-- 
        Viktor.

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