Mel P via Postfix-users:
> On 2026-05-25 13:08, Ralph Seichter via Postfix-users wrote:
> > * Mel P via Postfix-users:
> >
> >> From my own MX log data, I observe 20-25% of "legitimate" mailhosts
> >> connecting over IPv6. "Legitimate" here meaning those hosts that get
> >> past postscreen's bot countermeasures and DNSBLs.
> >
> > I wonder if that number might be skewed, to a degree? Servers with both
> > IPv6 and IPv4 connectivity need not use IPv6 as their first option when
> > trying to establish an outbound connection.
>
> The true number is probably higher.
>
> Postfix, for example, defaults to protocol balancing without preference.
> Without DNS correlation, a dual-stack emitter running postfix is
> indistinguishable from an IPv4-only emitter half the time.
>
> Looking at 1-25 May on my primary MX, 868 unique IPs sent 15.3k emails.
> Some details of that data:
>
> - 202 (23.4%) were IPv6;
> - 536 (61.7%) connected only once, 116 were IPv6;
> - 143 had PTRs in google.com, only 1 was IPv4;
> - 33 had PTRs in outlook.com, 3 were IPv4;
> - 12.8k of those emails came from mx2.freebsd.org, a dual-stack postfix
> emitter that connected via IPv6 50.3% of the time;
> - 579 of those emails came from GitHub, all from 36 IPs in 192.30.252.0/24.
>
> The observed Microsoft and Google emitters collectively being being
> 97.7% IPv6 suggests a very significant part of the internet's mail
> volume is sent from hosts that prefer IPv6 over IPv4.
>
> IMHO, that last reinforces the idea that it's time to retire the unique
> handling of inet_protocols, or at least change it to "inet_protocols = all"
That may be so for big providers, but what percentage of Postfix
sites has IPv6 connectivity? Turning on Postfix IPv6 support without
IPv6 connectivity would do no harm for the Postfix SMTP server, but
it would be harmul for the SMTP client.
Wietse
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