Re: Didn't receive the original post (to which this was a reply)

From	Joe <joe@jretrading.com>
Newsgroups	linux.debian.user
Subject	Re: Didn't receive the original post (to which this was a reply)
Date	2026-04-30 20:40 +0200
Message-ID	<MPEsN-1AoB-1@gated-at.bofh.it> (permalink)
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On Thu, 30 Apr 2026 13:57:32 -0400
Greg Wooledge <greg@wooledge.org> wrote:

> On Thu, Apr 30, 2026 at 18:02:25 +0100, Joe wrote:
> > Once SMTP servers started refusing connections from dodgy senders
> > (no PTR record, no complementary A/PTR pair, etc.) it was realised
> > that some SMTP servers would still accept the mail. Forging the
> > From: header to the real target of the spam would cause this person
> > not to be found by the gullible SMTP server, so it would bounce the
> > *entire* spam message 'back' to the alleged sender. The bounce came
> > from a 'legitimate' sender, so was accepted by the intended target
> > SMTP server and delivered to the recipient.  
> 
> For the record, this kind of attack involves falsifying the envelope
> sender address, not the From: header.  Bounce messages are sent to
> the envelope sender.  The From: header is strictly ornamental as far
> as SMTP is concerned, though some MTAs may use it in their spam
> detection heuristics, or in their client email acceptance policies.
> 

Sorry, yes, not quite awake. It's the Reply To: where the bounce is
sent, but if that isn't set in the headers, the envelope address is
copied to a new Reply To. A spammer makes sure the Reply To: is set.

The bounce mechanism is part of the message processing, after the SMTP
transaction has already accepted the message. A rejection during the
transaction simply terminates the SMTP handshake, sending an
explanatory message back to the actual sending IP address and never
actually receiving any of the message body or headers. There's no way to
route the rejection to anywhere other than the sender IP address, since
that's the other end of the transaction. The envelope sender address may
not resolve to the sender IP address, though it generally must resolve
in DNS.

A rejection is far better than a bounce, using less bandwidth and never
involving any other party. One of the main problems of twenty years or
so ago was than many businesses used an ISP to receive their email
which was then downloaded to the company server by POP3, or very
occasionally, IMAP4. Whatever the company server did, the ISP's server
had already accepted the email, so if the recipient didn't exist, the
company server had no option but to either bounce it or drop it.

-- 
Joe

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Moribund mailing list CGS <etphonehomefrance@gmail.com> - 2026-04-28 17:10 +0200
  Re: Moribund mailing list Norwid Behrnd <nbehrnd@yahoo.com> - 2026-04-28 17:30 +0200
    Re: Moribund mailing list "Jeremy Nicoll" <jn.ml.dbi.73@letterboxes.org> - 2026-04-28 19:20 +0200
      Re: Moribund mailing list David Wright <deblis@lionunicorn.co.uk> - 2026-04-28 20:20 +0200
        Re: Moribund mailing list "Jeremy Nicoll" <jn.ml.dbi.73@letterboxes.org> - 2026-04-28 20:40 +0200
          Re: Moribund mailing list Andy Smith <andy@strugglers.net> - 2026-04-28 21:50 +0200
          Re: Moribund mailing list David Wright <deblis@lionunicorn.co.uk> - 2026-04-29 01:00 +0200
            Re: Moribund mailing list "Jeremy Nicoll" <jn.ml.dbi.73@letterboxes.org> - 2026-04-29 01:40 +0200
  Re: Moribund mailing list Felix Miata <mrmazda@earthlink.net> - 2026-04-28 17:50 +0200
    Didn't receive the original post (to which this was a reply) (was: Re: Moribund mailing list) rhkramer@gmail.com - 2026-04-28 18:30 +0200
      Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Nicolas George <george@nsup.org> - 2026-04-28 19:30 +0200
        Re: Didn't receive the original post (to which this was a reply) (was: Re: Moribund mailing list) rhkramer@gmail.com - 2026-04-28 21:50 +0200
      Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Andy Smith <andy@strugglers.net> - 2026-04-28 20:10 +0200
        Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Jan Claeys <lists@janc.be> - 2026-04-28 21:20 +0200
          Re: Didn't receive the original post (to which this was a reply) nwe <nwe@gitcoding.net> - 2026-04-28 21:50 +0200
          Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Andy Smith <andy@strugglers.net> - 2026-04-28 21:50 +0200
            Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Joe <joe@jretrading.com> - 2026-04-28 23:00 +0200
              Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Nicolas George <george@nsup.org> - 2026-04-28 23:20 +0200
                Re: Didn't receive the original post (to which this was a reply) Nicolas George <george@nsup.org> - 2026-04-28 23:30 +0200
                Re: Didn't receive the original post (to which this was a reply) nwe <nwe@gitcoding.net> - 2026-04-29 00:00 +0200
                Re: Didn't receive the original post (to which this was a reply) Nicolas George <george@nsup.org> - 2026-04-29 11:10 +0200
                Re: Didn't receive the original post (to which this was a reply) nwe <nwe@gitcoding.net> - 2026-04-28 23:30 +0200
                Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Joe <joe@jretrading.com> - 2026-04-29 09:10 +0200
                Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Nicolas George <george@nsup.org> - 2026-04-29 11:10 +0200
              Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) <tomas@tuxteam.de> - 2026-04-29 12:00 +0200
              Re: Didn't receive the original post (to which this was a reply) (was: Re: Moribund mailing list) "Roy J. Tellason, Sr." <roy@rtellason.com> - 2026-04-29 18:50 +0200
                Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Nicolas George <george@nsup.org> - 2026-04-29 19:00 +0200
            Re: Didn't receive the original post (to which this was a reply) Stefan Monnier <monnier@iro.umontreal.ca> - 2026-04-28 23:20 +0200
              Re: Didn't receive the original post (to which this was a reply) Chris Green <cl@isbd.net> - 2026-04-29 08:30 +0200
                Re: Didn't receive the original post (to which this was a reply) Joe <joe@jretrading.com> - 2026-04-29 09:00 +0200
                Re: Didn't receive the original post (to which this was a reply) Stefan Monnier <monnier@iro.umontreal.ca> - 2026-04-30 17:50 +0200
                Re: Didn't receive the original post (to which this was a reply) Joe <joe@jretrading.com> - 2026-04-30 19:10 +0200
                Re: Didn't receive the original post (to which this was a reply) Greg Wooledge <greg@wooledge.org> - 2026-04-30 20:00 +0200
                Re: Didn't receive the original post (to which this was a reply) Joe <joe@jretrading.com> - 2026-04-30 20:40 +0200
                Re: Didn't receive the original post (to which this was a reply) Greg Wooledge <greg@wooledge.org> - 2026-04-30 21:20 +0200
              Re: Didn't receive the original post (to which this was a reply) CGS <etphonehomefrance@gmail.com> - 2026-04-30 16:50 +0200
            Google employee expertise (was: Re: Didn't receive the original post (to which this was a reply) (was: Re: Moribund mailing list)) rhkramer@gmail.com - 2026-04-30 16:00 +0200
              Re: Google employee expertise (was: Re: Didn't receive the original post (to which this was a reply) (was: Re: Moribund mailing list)) rhkramer@gmail.com - 2026-05-01 02:30 +0200
                Re: Google employee expertise (was: Re: Didn't receive the original  post (to which this was a reply) Thomas Dineen <tdineen2021@gmail.com> - 2026-05-01 22:20 +0200
                On tolerance [was: Google employee expertise ...] <tomas@tuxteam.de> - 2026-05-02 07:40 +0200
      Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Jeffrey Walton <noloader@gmail.com> - 2026-04-28 21:10 +0200
        Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Joe <joe@jretrading.com> - 2026-04-28 21:30 +0200
      Re: Didn't receive the original post (to which this was a reply)  (was: Re: Moribund mailing list) Anders Andersson <pipatron@gmail.com> - 2026-04-28 21:30 +0200
      Re: Didn't receive the original post (to which this was a reply) Richmond <dnomhcir@gmx.com> - 2026-04-28 22:20 +0200
        Re: Didn't receive the original post (to which this was a reply) rhkramer@gmail.com - 2026-04-28 22:30 +0200
  Re: Moribund mailing list Radhitya <alif@radhitya.org> - 2026-05-01 15:50 +0200

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