postfix-master/CONTENT_INSPECTION_README.html

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   8 <title>Postfix Content Inspection </title>
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  16 <h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix
  17 Content Inspection </h1>
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  20
  21 <p> Postfix supports three content inspection methods, ranging from
  22 light-weight one-line-at-a-time scanning before mail is queued, to
  23 heavy duty machinery that does sophisticated content analysis after
  24 mail is queued. Each approach serves a different purpose.  </p>
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  26 <dl>
  27
  28 <dt> <b> before queue, built-in, light-weight</b> </dt>
  29
  30 <dd> <p> This method inspects mail BEFORE it is stored in the queue,
  31 and uses Postfix's built-in message header and message body
  32 inspection. Although the main purpose is to stop a specific flood
  33 of mail from worms or viruses, it is also useful to block a flood
  34 of bounced junk email and email notifications from virus detection
  35 systems.  The built-in regular expressions are not meant to implement
  36 general SPAM and virus detection. For that, you should use one of
  37 the content inspection methods described below. Details are described
  38 in the <a href="BUILTIN_FILTER_README.html">BUILTIN_FILTER_README</a> and <a href="BACKSCATTER_README.html">BACKSCATTER_README</a> documents.
  39 </p>
  40
  41 <dt> <b> after queue, external, heavy-weight</b> </dt>
  42
  43 <dd> <p> This method inspects mail AFTER it is stored in the queue,
  44 and uses standard protocols such as SMTP or "pipe to command and
  45 wait for exit status".  After-queue inspection allows you to use
  46 content filters of arbitrary complexity without causing timeouts
  47 while receiving mail, and without running out of memory resources
  48 under a peak load. Details of this approach are in the <a href="FILTER_README.html">FILTER_README</a>
  49 document. </p>
  50
  51 <dt> <b> before queue, external, medium-weight</b> </dt>
  52
  53 <dd> <p> The following two methods inspect mail BEFORE it is stored in the
  54 queue.  </p>
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  56 <ul>
  57
  58 <li> <p> The first method uses the SMTP protocol, and is described
  59 in the <a href="SMTPD_PROXY_README.html">SMTPD_PROXY_README</a> document.  This approach is available
  60 with Postfix version 2.1 and later.  </p>
  61
  62 <li> <p> The second method uses the Sendmail 8 Milter protocol, and
  63 is described in the <a href="MILTER_README.html">MILTER_README</a> document.  This approach is
  64 available with Postfix version 2.3 and later.  </p>
  65
  66 </ul>
  67
  68 <p> Although these approaches appear to be attractive, they have
  69 some serious limitations that you need to be aware of.  First,
  70 content inspection software must finish in a limited amount of time;
  71 if content inspection needs too much time then incoming mail
  72 deliveries will time out.  Second, content inspection software must
  73 run in a limited amount of memory; if content inspection needs too
  74 much memory then software will crash under a peak load.  Before-queue
  75 inspection limits the peak load that your system can handle, and
  76 limits the sophistication of the content filter that you can use.
  77 </p>
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  80
  81 <p> The more sophisticated content filtering software is not built
  82 into Postfix for good reasons: writing an MTA requires different
  83 skills than writing a SPAM or virus killer. Postfix encourages the
  84 use of external filters and standard protocols because this allows
  85 you to choose the best MTA and the best content inspection software
  86 for your purpose.  Information about external content inspection
  87 software can be found on the Postfix website at <a href="http://www.postfix.org/">http://www.postfix.org/</a>,
  88 and on the postfix-users@postfix.org mailing list. </p>
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