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

John Levine <johnl@taugh.com> Sun, 27 September 2020 05:22 UTC

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Date: 27 Sep 2020 01:22:21 -0400
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From: "John Levine" <johnl@taugh.com>
To: ietf-smtp@ietf.org
Cc: moore@network-heretics.com
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Subject: Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321
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In article <da460777-824b-1f13-be7c-32bfa9664d02@network-heretics.com> you write:
>On 9/18/20 6:36 PM, Sam Varshavchik wrote:
>>
>> Courier has an optional setting that can be enabled, that verifies 
>> that "the domain name argument in the EHLO command actually correspond 
>> to the IP address". I have it enabled. It's one of my most successful 
>> spam filters. ...

>At some point in the past, this was _not_ a reliable spam filter. ...

I think you may be conflating SMTP and submission. For submission,
you're right, the EHLO argument is frequently some random name that a
computer thinks it has behind a couple of layers of NAT. For SMTP,
server to server, I agree with Sam that it is extremely rare for a
legit message to come from a host that doesn't know its name.

R's,
John