Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321

Sam Varshavchik <> Sun, 27 September 2020 12:58 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 844E83A0EFF for <>; Sun, 27 Sep 2020 05:58:35 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -1.9
X-Spam-Status: No, score=-1.9 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id BX2duQA0MIeT for <>; Sun, 27 Sep 2020 05:58:34 -0700 (PDT)
Received: from ( []) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by (Postfix) with ESMTPS id 5AD373A0F00 for <>; Sun, 27 Sep 2020 05:58:34 -0700 (PDT)
Received: from ( [::ffff:]) (TLS: TLSv1.3,256bits,TLS_AES_256_GCM_SHA384) by with UTF8ESMTPS id 00000000002C0020.000000005F708C78.000030D6; Sun, 27 Sep 2020 08:58:32 -0400
Received: from (localhost []) (IDENT: uid 1004) by with UTF8SMTP id 000000000001C7C3.000000005F708C78.000060C3; Sun, 27 Sep 2020 08:58:32 -0400
References: <> <>
Message-ID: <>
From: Sam Varshavchik <>
Date: Sun, 27 Sep 2020 08:58:31 -0400
Mime-Version: 1.0
X-Mime-Autoconverted: from 8bit to quoted-printable by mimegpg
Content-Type: multipart/signed; boundary=""; micalg=pgp-sha1; protocol="application/pgp-signature"
Archived-At: <>
Subject: Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "Discussion of issues related to Simple Mail Transfer Protocol \(SMTP\) \[RFC 821, RFC 2821, RFC 5321\]" <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Sun, 27 Sep 2020 12:58:36 -0000

Keith Moore writes:

> 3. 821, 1123, and subsequent revisions all seem to be based on the  
> assumption that if you're operating an SMTP server, you're trying in good  
> faith to deliver (legitimate) email reliably.   I'm not sure this assumption

And you should have a pretty good idea of what your IP address is.
> Seen from that perspective, maybe 5321's language about EHLO arguments could  
> use some updating along the following lines:
> - For a very many reasons [which could be listed, or not], SMTP servers have  
> no reasonable expectation of being able to determine the validity or  
> legitimacy of a message based on comparison of the EHLO command argument  
> with anything else at all.   Therefore if what you're trying to do is  
> reliably deliver legitimate mail (for some meaning of legitimate),  
> validation of EHLO arguments is useless and strongly NOT RECOMMENDED.

The exact phraseology is only secondary. The point I was making is that I  
see that EHLO/HELO validation is employed in practice, and it is in  
practical use. And based on my own experience, it is highly effective. Like  
I said, in 20+ years I've been doing strict domain validation on HELO/EHLO I  
do not recall a single false positive, and a mind-boggling amount of crap  
that got blocked.

And I think that in practical situations this is going to outrank, in  
peoples' minds, any demand that they MUST NOT do that.

> Of course, if your goal is really to discard mail for no good reason, and  
> you're not handling incoming mail for anyone but yourself, have at it!    
> Just have the decency to blackhole the mail rather than bounce it, since  
> you're really not doing anyone any favors.

On that point I'll also have to disagree. It's better to reject the mail  
with a 5xx, than /dev/null it.