The first new parameter is called
specifies what protocols Postfix will use when it makes or accepts
network connections, and also controls what DNS lookups Postfix
will use when it makes network connections.
# You must stop/start Postfix after changing this parameter.
inet_protocols = ipv4 (DEFAULT: enable IPv4 only)
inet_protocols = all (enable IPv4, and IPv6 if supported)
inet_protocols = ipv4, ipv6 (enable both IPv4 and IPv6)
inet_protocols = ipv6 (enable IPv6 only)
By default, Postfix uses IPv4 only, because most systems aren't
attached to an IPv6 network.
On systems with combined IPv4/IPv6 stacks, attempts to
deliver mail via IPv6 would always fail with "network unreachable",
and those attempts would only slow down Postfix.
Linux kernels don't even load IPv6 protocol support by
default. Any attempt to use it would fail immediately.
Note 1: you must stop and start Postfix after changing the
inet_protocols configuration parameter.
Note 2: if you see error messages like the following, then
you're running Linux and need to turn on IPv6 in the kernel: see
https://www.ipv6.org/ for hints and tips. Unlike other systems,
Linux does not have a combined stack for IPv4 and IPv6, and IPv6
protocol support is not loaded by default.
inet_protocols: IPv6 support is disabled: Address family not supported by protocol
inet_protocols: configuring for IPv4 support only
Note 3: on older Linux and Solaris systems, the setting
inet_protocols = ipv6" will not prevent Postfix from
accepting IPv4 connections. Postfix will present the client IP
addresses in IPv6 format, though. In all other cases, Postfix always
presents IPv4 client IP addresses in the traditional dotted quad