[pfx] Re: Should the configuration upgrade process still append "inet_protocols = ipv4"?

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.
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