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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.

bbeeffoorree qquueeuuee,, bbuuiilltt--iinn,, lliigghhtt--wweeiigghhtt
    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.

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    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.

bbeeffoorree qquueeuuee,, eexxtteerrnnaall,, mmeeddiiuumm--wweeiigghhtt
    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 postfix-users@postfix.org mailing list.