If a mail server relays a message to another domain (that is, the recipient's address is not in our domain) and the message originates from outside our network, the server is known as an open relay. Spammers are constantly on the lookout for open relays because such permissive behavior is easy to take advantage of, and Postfix tries to protect us by default by only relaying messages that come from our network. Unfortunately, it's not practical to restrict legitimate users from sending e-mail through the server only when they're on our network. This recipe teaches you how to add Simple Authentication and Security Layer (SASL) authentication to Postfix's configuration using Dovecot. Postfix will then happily relay messages for our users authenticated users, regardless of their network location, while still refusing to do so for anyone else.