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Postfix: The Definitive Guide by Kyle D. Dent

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Daemon-Based Filtering

Daemon-based filtering offers a more advanced architecture over the command-based method with lower cost in I/O and CPU usage. It can provide better error handling than is possible with the command method. If implemented as a resident process, the startup overhead per message is eliminated. A daemon-based content filter can pass email messages back and forth with Postfix using the standard SMTP or LMTP protocol. Such a filter can run as a standalone daemon or it can be started by Postfix if configured to do so in master.cf.

In this configuration, we want the content filter to handle all messages, whether delivered locally (via sendmail) or to the smtpd daemon. You have to configure Postfix in master.cf to use a special smtp client component to deliver the messages to your filter and an additional smtpd daemon to receive messages back from your filter. Figure 14-2 illustrates how a filtered message travels through Postfix to your content filter and back into Postfix for delivery. In this diagram, the filter receives mail via localhost port 10025 from the additional smtp client and submits it back to Postfix via localhost port 10026 to the additional smtpd server component.

Mail-filtering daemon

Figure 14-2. Mail-filtering daemon

If the filter wants to reject a message, it should reply with an SMTP code of 550 along with the reason for the rejection. Otherwise, it should accept the message ...

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