I have a server on a LAN that need to send emails. Of course, this server has access to the internet through NAT, but for emails it is a bit more difficult : nowadays, email servers are checking many things when receiving emails : reverse, ‘from’ field, etc. and you may not have access to this kind of tuning for the internet IP address used with your NAT. So I needed to configure a relayhost, with direct access to the internet, that would accept my incoming email and relay it through normal channels. But I don’t want this server to be an open relay, so I have to setup authentication between them.