15.2.1 About SMTP

The Simple Mail Transfer Protocol (SMTP) is a transport protocol that provides mail delivery services between email client applications and servers, and between the originating server and the destination server. You must specify the SMTP server when you configure outgoing email for an email client application.

SMTP does not require authentication. Anyone can use SMTP to send email, including junk email and unsolicited bulk email. If you administer an SMTP server, you can configure relay restrictions that limit users from sending email through it. Open relay servers do not have any such restrictions. Both Postfix and Sendmail are SMTP server programs that use SMTP. Unless you own a domain in which you want to receive email, you do not need to set up an SMTP server.