From owner-freebsd-current  Fri Jul 25 00:12:26 1997
Return-Path: <owner-freebsd-current>
Received: (from root@localhost)
          by hub.freebsd.org (8.8.5/8.8.5) id AAA18826
          for current-outgoing; Fri, 25 Jul 1997 00:12:26 -0700 (PDT)
Received: from mercury.Sun.COM (mercury.Sun.COM [192.9.25.1])
          by hub.freebsd.org (8.8.5/8.8.5) with SMTP id AAA18788
          for <current@FreeBSD.ORG>; Fri, 25 Jul 1997 00:12:18 -0700 (PDT)
Received: from East.Sun.COM ([129.148.1.241]) by mercury.Sun.COM (SMI-8.6/mail.byaddr) with SMTP id AAA27791; Fri, 25 Jul 1997 00:11:47 -0700
Received: from suneast.East.Sun.COM by East.Sun.COM (SMI-8.6/SMI-5.3)
	id DAA27433; Fri, 25 Jul 1997 03:11:44 -0400
Received: from compound.east.sun.com by suneast.East.Sun.COM (SMI-8.6/SMI-SVR4)
	id DAA15751; Fri, 25 Jul 1997 03:11:44 -0400
Received: (from alk@localhost) by pobox.com (8.8.6/8.7.3) id OAA03645; Thu, 24 Jul 1997 14:28:32 -0500 (CDT)
Date: Thu, 24 Jul 1997 14:28:32 -0500 (CDT)
Reply-To: Anthony.Kimball@East.Sun.COM
Message-Id: <199707241928.OAA03645@pobox.com>
From: Tony Kimball <Anthony.Kimball@East.Sun.COM>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
To: jflists@calweb.com
Cc: Anthony.Kimball@East.Sun.COM, current@FreeBSD.ORG
Subject: Re: (over)zealous mail bouncing 
References: <199707241422.HAA00957@hub.freebsd.org>
	<199707241601.LAA03086@compound.east.sun.com>
	<3.0.3.32.19970724103920.0094c100@pop.calweb.com>
X-Face: O9M"E%K;(f-Go/XDxL+pCxI5<W_e*r7B8ojMD.5+qDgOp4/\gTQEgbOoBqf:Vtl*aW:<;9/
	UIVk'U^XhYg+v#r~*X<$Q=_ik@5:%E}pl]9|9|:]WLW8n:fLgg0~cfwWRBQxK/HSG0b1D/gljJC$qp
	b=HDF@+"H<|fXy`w#$8#6A"!F:>*gr[=FN@Y`cl1.Tn
Sender: owner-freebsd-current@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk

Quoth jfesler@calweb.com on Thu, 24 July:
: 
: We actively block any SMTP session where the MAIL FROM: command  lacks a
: valid DNS entry.  

Okay, this is another issue entirely.  You are requiring that every
piece of mail have a return path out to the Internet leaf node, yes?
(Coming to an understanding with one correspondent evidently does
not mean that one has come to understand all correspondents!)
That does not guarantee a return path, because there may be further
transports beyond the edge of the Internet, but probably gets you one
in the vast majority of cases.

As I understand it, you are only bouncing mail which cannot be replied
*through* a valid MX or A.  I'm guessing that you could probably
construct a valid address from the various "Received:" headers in many
such cases, though.

I suppose this will help to reject casual spam, but it seems to me a
misdirected effort, since most spam, particularly the large-volume
stuff from the pros, will not get filtered in this way.  The only
way I can see to do that is to maintain a large kill list.