In the previous state, postfix would still try to use IPv6 addresses,
even when it is disabled in the global networking config.
Cf. https://www.postfix.org/postconf.5.html:
With Postfix 2.8 and earlier the default is "ipv4". For backwards compatibility with these releases,
the Postfix 2.9 and later upgrade procedure appends an explicit "inet_protocols = ipv4" setting to
main.cf when no explicit setting is present.
This compatibility workaround will be phased out as IPv6 deployment becomes more common.
inet_protocols = ipv4
inet_protocols = all (DEFAULT)
inet_protocols = ipv6
inet_protocols = ipv4, ipv6
So setting it to 'all' conditionally does not help, as we are now on version 3.x.
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