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

John Levine <johnl@taugh.com> Mon, 28 September 2020 14:28 UTC

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Date: 28 Sep 2020 10:27:58 -0400
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From: "John Levine" <johnl@taugh.com>
To: ietf-smtp@ietf.org
Cc: laura@wordtothewise.com
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Subject: Re: [ietf-smtp] EHLO domain validation requirement in RFC 5321
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In article <0EB43DCF-727B-496E-A92E-DE2BC685E026@wordtothewise.com> you write:
>NATs, yes. NATs handling significant amounts of SMTP traffic, I’ve not seen any evidence for.

I've seen lots of CGNs handling lots of consumer traffic. Many mobile
networks are IPV6 only, with NAT64 to talk to the IPv4 Internet. I
have seen no servers at all behind NATs and I don't ever exoect to see
any. Servers need static addresses so clients can find them, so if
they're visible on IPv4 at all, they have native IPv4 addresses. Every
outbound mail server is associated with an inbound mail server, so if
they can find IPv4 addresses for the inbound server, they can also
find some for the outbound server.

Keith is asking us to expect that mail clients will move behind NAT64
even while their associated servers do not, and that will happen
enough that it will change a strong spam filtering signal. Doesn't
seem likely to me.

R's,
John