Posted by Helder Guerreiro

The file master.cf controls the functioning of the Postfix server processes.
With this file one may configure Postfix to filter or do something else to the mail that comes and goes from the system.

This file format is available on the manual master (5), and you can check it with the command:

$ man 5 master

If you don't have it installed on your system you can google for it.

Lets make an example.
Imagine that you have a system with the IMAP Cyrus server installed and you want to filter the emails with Spamassassin, before they are delivered to the final user mail box.
You can do the following:

• In the file /etc/postfix/main.cf state that the received mails transport to use is:

  mailbox_transport=spamcyrus

• Then, to finish, state on the file /etc/postfix/master.cf the treatment to be applied:

  spamcyrus unix  -       n       n       -       -       pipe
    user=cyrus argv=/usr/bin/spamc -e
    /usr/sbin/cyrdeliver -e -r ${sender} -m ${extension} ${user}

Please note that the lines starting with a space are assumed to be part of the previous line, meaning that this configuration is done in a single line.

What happens next, once an email is received, is that the email is send to the spamc stdin (the spamassassin deamon client) and this one sends the email already treated to the Cyrus delivery proccess.

Using this configuration one avoids the use of Amavis, that is normally used to do this kind of treatment.