Postfix, by default only listen on ipv4 interface.
this is howto make Postfix listen on both ipv4/ipv6
This example using IPv6 address given by he.net

In main.cf

-- others config --
-- snippet --
inet_protocols = ipv4, ipv6
inet_interfaces = 127.0.0.1, 192.168.200.18, [::1], [2001:470:19:xxxx::2]

mynetworks = [2001:470:19:xxxx::/64], [::1/128], 127.0.0.1, 192.168.200.0/24

-- others config --
-- end of snippet --

easy isn’t it? :) IPv6 is enabled out of the box in postfix, all you have to is little configuration tweak.
Have a nice try ;)

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