Postfix: The Definitive Guide

Command-Based Filtering

The simplest way to set up content filtering is to use a program that runs as a command and accepts the contents of a message on its standard input. Postfix delivers messages to your filter command via the pipe mailer. Your filter command performs its checking and then gives the filtered message back to Postfix using the Postfix sendmail command.

For this discussion, we’ll assume that the filter command operates on mail that comes in through the SMTP daemon but not on mail that is delivered locally (using the sendmail command), so that your filter can use sendmail to give the message back to Postfix without looping. Figure 14-1 illustrates the path messages follow once you put your filter in place. Rather than passing the message to a delivery agent, the queue manager invokes the filter.

Figure 14-1. Mail-filtering command

Your filter program must be able to accept the message on its standard input and then deliver it to the Postfix sendmail command. If you have a filtering program that doesn’t handle input and output in this way it should be easy enough to create a shell script wrapper to deal with those details. In the Postfix distribution, the FILTER_README file contains an example of such a script.

Configuration

When you configure Postfix to use your filter program, you must specify a user that the program runs as. You should create a pseudoaccount whose ...