Mel P via Postfix-users:
> On 2026-05-27 10:50, Wietse Venema via Postfix-users wrote:
> > Ralph Seichter via Postfix-users:
> >> * Wietse Venema via Postfix-users:
> >>> Mel P via Postfix-users:
> >>>
> >>>> 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.
> >>
> >> I might be missing something, but how would the SMTP server supporting
> >> both IPv4 and IPv6 by default be harmful for the SMTP client? And what
> >> is "the" SMTP client in the case you envision?
> >
> > Turning on Postfix IPv6 support without IPv6 connectivity would do
> > no harm for the Postfix SMTP server.
> >
> > Turning on Postfix IPv6 support without IPv6 connectivity would be
> > harmul for the Postfix SMTP client.
>
> IME worst case there's a few seconds delay when there's a partial
> network outage, but fallback is otherwise reliably seamless and
> instantaneous.
You are missing one thing.
The Postfix SMTP client will try only a limited number of IP
addresses, configured with smtp_mx_address_limit (default: 5).
This limit ensures that a receiver cannot get an unfair amount
of Postfix SMTP client resources.
When a receiver lists a lot of IPv6 addresses that Postfix cannot
use, then smtp_balance_inet_protocols tries to ensure that there
will be ~2 IPv4 addresses. That is less than 5.
The harm done then, is that Postfix will have fewer opportunities
to retry delivery. And depending on how exactly IPv6 is broken, the
failing connections my haver to time out.
Wietse
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