Re: [ietf-smtp] MTS-STS validation when MX host points to a CNAME, violating RFC 2181 § 10.3

John Levine <johnl@taugh.com> Fri, 02 April 2021 00:24 UTC

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Date: 1 Apr 2021 20:24:15 -0400
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From: "John Levine" <johnl@taugh.com>
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Subject: Re: [ietf-smtp] =?utf-8?q?MTS-STS_validation_when_MX_host_points_to_?= =?utf-8?q?a_CNAME=2C_violating__RFC_2181_=C2=A7_10=2E3?=
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It appears that Sam Varshavchik  <mrsam@courier-mta.com> said:
>My viewpoints are somewhat dated. The persona-non-grata status of MXs  
>pointing to CNAMEs – that verbiage appears to be new to the current SMTP  
>standard, I checked and did not find any equivalent language in the prior  
>one; so my implementation reflected that.

The author of 5321 is on this list so perhaps he recalls what the motivation
for the change was.  I'm sure it wasn't accidental.