SMTP and sendmail
The Simple Mail Transfer Protocol (SMTP) is the established standard for transferring mail over the Internet. The sendmail program provides the services needed to support SMTP connections for Linux.
This section covers the details you need in order to understand, install, and configure the sendmail package. Before getting into the details, however, let's take a moment to discuss the SMTP protocol in better detail and how the Domain Name Service (DNS) interacts with email across the Internet. (See Chapter 14, "Domain Name Service and Dynamic Host Configuration Protocol," for more details on DNS configuration.)
Armed with a better understanding of the protocols, you can take on trying to understand sendmail itself, beginning with ...