Postfix supports three content inspection methods, ranging from light-weight one-line-at-a-time scanning before mail is queued, to heavy duty machinery that does sophisticated content analysis after mail is queued. Each approach serves a different purpose.
This method inspects mail BEFORE it is stored in the queue, and uses Postfix's built-in message header and message body inspection. Although the main purpose is to stop a specific flood of mail from worms or viruses, it is also useful to block a flood of bounced junk email and email notifications from virus detection systems. The built-in regular expressions are not meant to implement general SPAM and virus detection. For that, you should use one of the content inspection methods described below. Details are described in the BUILTIN_FILTER_README and BACKSCATTER_README documents.
This method inspects mail AFTER it is stored in the queue, and uses standard protocols such as SMTP or "pipe to command and wait for exit status". After-queue inspection allows you to use content filters of arbitrary complexity without causing timeouts while receiving mail, and without running out of memory resources under a peak load. Details of this approach are in the FILTER_README document.
The following two methods inspect mail BEFORE it is stored in the queue.
The first method uses the SMTP protocol, and is described in the SMTPD_PROXY_README document. This approach is available with Postfix version 2.1 and later.
The second method uses the Sendmail 8 Milter protocol, and is described in the MILTER_README document. This approach is available with Postfix version 2.3 and later.
Although these approaches appear to be attractive, they have some serious limitations that you need to be aware of. First, content inspection software must finish in a limited amount of time; if content inspection needs too much time then incoming mail deliveries will time out. Second, content inspection software must run in a limited amount of memory; if content inspection needs too much memory then software will crash under a peak load. Before-queue inspection limits the peak load that your system can handle, and limits the sophistication of the content filter that you can use.
The more sophisticated content filtering software is not built into Postfix for good reasons: writing an MTA requires different skills than writing a SPAM or virus killer. Postfix encourages the use of external filters and standard protocols because this allows you to choose the best MTA and the best content inspection software for your purpose. Information about external content inspection software can be found on the Postfix website at http://www.postfix.org/, and on the firstname.lastname@example.org mailing list.