Wietse Venema via Postfix-users:
> It's a rather long explanation for "why not do X". like several
> times longer than the text that explains what protocol preferences
> do. And this is the only place where adding that text would help.
I updated the text a little:
Notes for mail delivery between sites that have both IPv4 and IPv6 connectivity:
* The setting "smtp_address_preference = ipv6" is unsafe. All
deliveries will suffer delays when IPv6 is not available even
while the destination is still reachable over IPv4. Mail may
be stuck in the queue with Postfix versions < 3.3 that do
not implement "smtp_balance_inet_protocols". For similar reasons,
the setting "smtp_address_preference = ipv4" is also unsafe.
* The setting "smtp_address_preference = any" is safe. With this,
and "smtp_balance_inet_protocols = yes" (the default), only
half of deliveries will suffer delays if there is an outage
that affects IPv6 or IPv4, as long as it does not affect both.
I added smtp_balance_inet_protocols to the discussion, because it
mitigates the safety issue somewhat, perhaps to the point that some
people with low email volume are willing to suffer the delays.
Wietse
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