6.1 Postfix Architecture

Several different Postfix components play roles in receiving messages from the Internet. The master daemon is responsible for the coordination of the components. Messages from the Internet typically enter the mail server via the smtpd daemon, which listens on port 25 and conducts the SMTP transaction with the remote sender. smtpd passes each message to the cleanup daemon, which performs sanity checks, fixes missing headers, and (with the help of the trivial-rewrite program) rewrites addresses. cleanup then deposits each message in the incoming mail queue and alerts the qmgr daemon. qmgr moves messages from the incoming queue to the active queue, and then calls local for delivery to local recipients (or smtp for relaying to remote recipients by SMTP). Figure 6-1 illustrates the flow of email through Postfix components.

Figure 6-1. The Postfix architecture during message receipt

Most systems keep Postfix's configuration files in /etc/postfix . The most important files are main.cf , which contains nearly all of the configuration directives for Postfix, and master.cf , which configures the master daemon and determines how the various Postfix components will be run. After you make changes to either of these files, you should issue the postfix reload command to cause Postfix to re-read the files.