Mail delivery from a client application to the server, and from an originating server to the destination server is handled by the Simple Mail Transfer Protocol (SMTP) .
The primary purpose of SMTP is to transfer email between mail servers. However, it is critical for email clients as well. In order to send email, the client sends the message to an outgoing mail server, which in turn contacts the destination mail server for delivery. For this reason, it is necessary to specify an SMTP server when conguring an email client. Under Red Hat Linux, a user can congure an SMTP server on the local machine to handle mail delivery. However, it is also possible to congure remote SMTP servers for outgoing mail. One important point to make about the SMTP protocol is that it does not require authentication. This allows anyone on the Internet to send email to anyone else or even to large groups of people. It is this characteristic of SMTP that makes junk email or spam possible. Modern SMTP servers attempt to minimize this behavior by allowing only known hosts access to the SMTP server. Those servers that do not impose such restrictions are called open relay servers.
Red Hat Linux uses Sendmail (/usr/sbin/sendmail) as its default SMTP program. However, a
simpler mail server application called Postx (/usr/sbin/postfix) is also available.