On 2026-05-27 12:08, Wietse Venema via Postfix-users wrote:
Mel P via Postfix-users:
On 2026-05-27 10:50, Wietse Venema via Postfix-users wrote:
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.
I did overlook that, but I would point out that the reverse is also
true. If a receiver lists a lot IPv4 addresses, but the sender has IPv4
reachability issues, the usable IPv6 addresses will similarly be
supressed. That case is undoubtedly quite rare, but I have encountered
fixed cellular ISPs where IPv6 is native, but IPv4 uses CGNAT for all
customers.
All of this is merely IMO, of course. I personally would love to see
the handling of inet_protocols changed to use compatibility_level at
least, but I can also see the impetus to keep it as is.
_______________________________________________
Postfix-users mailing list -- [email protected]
To unsubscribe send an email to [email protected]
