I have a Mac running OS X Sierra Server (5.3.1). The Mail section is a bit dumbed down on previous versions. There is no GUI option to 'Allow relaying from:'. I believe this is actually Postfix and that with Postfix you allow relaying in the main.cf 'mynetworks' section. There are three postfix/main.cf files: /Applications/Server.app/Contents/ServerRoot/Library/Server/Mail/Config/postfix/main.cf /Library/Server/Mail/Config/postfix/main.cf /private/etc/postfix/main.cf I have edited all of them but still no luck. Anyone know what I am doing wrong?