Postfix IPv4 Only

I have a postfix SMTP relay buried deep in a network behind proxy servers, all the infrastructure [sadly] is IPv4 only. This works, yet one ends up with many log messages like:

connect to smtp.office365.com[2603:1036:304:2857::2]:587: Network is unreachable

The server attempts if IPv6 result from the DNS lookup first. So let's make postfix use IPv4 only.

postconf -e inet_protocols=ipv4

That's it! No more "unreachable" log messages.

BTW, the default value of inet_protocols is "all". Set it back to that value to re-enable IPv6.