dist/proto/CONTENT_INSPECTION_README.html

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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix
Content Inspection </h1>

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<p> 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.  </p>

<dl>

<dt> <b> before queue, built-in, light-weight</b> </dt>

<dd> <p> 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.
</p>

<dt> <b> after queue, external, heavy-weight</b> </dt>

<dd> <p> 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. </p>

<dt> <b> before queue, external, medium-weight</b> </dt>

<dd> <p> The following two methods inspect mail BEFORE it is stored in the
queue.  </p>

<ul>

<li> <p> 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.  </p>

<li> <p> 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.  </p>

</ul>

<p> 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.
</p>

</dl>

<p> 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 https://www.postfix.org/,
and on the postfix-users (a] postfix.org mailing list. </p>

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