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.built-in, light-weight, real-time
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.external, heavy-weight, not real time
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.external, medium-weight, real-time
This method inspects mail BEFORE it is stored in the queue, and uses the SMTP protocol. Although this approach appears to be the more attractive one, it really combines the worst of the other two. Because mail is inspected before it is queued, content inspection software must finish in a limited amount of time, and must run in a limited amount of memory. If content inspection needs too much time then incoming mail deliveries will time out, and 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. Details are in the SMTPD_PROXY_README document. This approach is available only with Postfix version 2.1 and later.
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.