proxy_interfaces (default: empty)
The network interface addresses that this mail system receives mail on by way of a proxy or network address translation unit. This feature is available in Postfix 2.0 and later.
You must specify your “outside” proxy/NAT addresses when your system is a backup MX host for other domains, otherwise mail delivery loops will happen when the primary MX host is down.
proxy_interfaces = 188.8.131.52
If you miss this step, you’ll see messages like this in your log file:
Apr 8 20:10:14 hades postfix/smtp: 08CABB59: to=