Ubuntu Unleashed 2015 Edition: Covering 14.10 and 15.04, Tenth Edition

Using Smart Hosts

If you do not have a full-time connection to the Internet, you will probably want to have Postfix send your messages to your ISP’s mail server and let it handle delivery for you. Without a full-time Internet connection, you could find it difficult to deliver messages to some locations (such as some underdeveloped areas of the world where email services are unreliable and sporadic). In those situations, you can configure Postfix to function as a smart host by passing email on to another sender instead of attempting to deliver the email directly. You can use a line such as the following in the main.cf file to enable a smart host:

relayhost = mail.isp.net

This line causes Postfix to pass any mail it receives to the server mail.isp.net ...