Some mail servers are connected to the Internet via a network
address translator (NAT) or proxy. This means that systems on the
Internet connect to the address of the NAT or proxy, instead of
connecting to the network address of the mail server. The NAT or
proxy forwards the connection to the network address of the mail
server, but Postfix does not know this.
If you run a Postfix server behind a proxy or NAT, you need to
proxy_interfaces parameter and specify all the external
proxy or NAT addresses that Postfix receives mail on. You may
specify symbolic hostnames instead of network addresses.
IMPORTANT: You must specify your proxy/NAT external 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.
Example: host behind NAT box running a backup MX host.
proxy_interfaces = 184.108.40.206 (the proxy/NAT external network address)