Postfix and SSL Postfix is one of the most popular e-mail servers after Sendmail. It benefits from most of Sendmail's features, but is a lot easier to install and maintain. Where Sendmail uses m4 files for configuration, Postfix uses the familiar key=value configuration files. SSL provides a way to encrypt a connection based on certificates. For more information on SSL and certificates, make sure you read Introduction to SSL on the site. Postfix and SSL can provide a way to allow relaying from dynamic IP addresses (road warriors). It bases its decision on whether to allow the client to relay if the certificate offered by the client is registered in a Postfix hash table. It also encrypts the channel between the client and the server, providing that little bit of extra security.