1 <!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN" 2 "https://www.w3.org/TR/html4/loose.dtd"> 3 4 <html> 5 6 <head> 7 8 <title>Postfix Content Inspection </title> 9 10 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 11 <link rel='stylesheet' type='text/css' href='postfix-doc.css'> 12 13 </head> 14 15 <body> 16 17 <h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix 18 Content Inspection </h1> 19 20 <hr> 21 22 <p> Postfix supports three content inspection methods, ranging from 23 light-weight one-line-at-a-time scanning before mail is queued, to 24 heavy duty machinery that does sophisticated content analysis after 25 mail is queued. Each approach serves a different purpose. </p> 26 27 <dl> 28 29 <dt> <b> before queue, built-in, light-weight</b> </dt> 30 31 <dd> <p> This method inspects mail BEFORE it is stored in the queue, 32 and uses Postfix's built-in message header and message body 33 inspection. Although the main purpose is to stop a specific flood 34 of mail from worms or viruses, it is also useful to block a flood 35 of bounced junk email and email notifications from virus detection 36 systems. The built-in regular expressions are not meant to implement 37 general SPAM and virus detection. For that, you should use one of 38 the content inspection methods described below. Details are described 39 in the BUILTIN_FILTER_README and BACKSCATTER_README documents. 40 </p> 41 42 <dt> <b> after queue, external, heavy-weight</b> </dt> 43 44 <dd> <p> This method inspects mail AFTER it is stored in the queue, 45 and uses standard protocols such as SMTP or "pipe to command and 46 wait for exit status". After-queue inspection allows you to use 47 content filters of arbitrary complexity without causing timeouts 48 while receiving mail, and without running out of memory resources 49 under a peak load. Details of this approach are in the FILTER_README 50 document. </p> 51 52 <dt> <b> before queue, external, medium-weight</b> </dt> 53 54 <dd> <p> The following two methods inspect mail BEFORE it is stored in the 55 queue. </p> 56 57 <ul> 58 59 <li> <p> The first method uses the SMTP protocol, and is described 60 in the SMTPD_PROXY_README document. This approach is available 61 with Postfix version 2.1 and later. </p> 62 63 <li> <p> The second method uses the Sendmail 8 Milter protocol, and 64 is described in the MILTER_README document. This approach is 65 available with Postfix version 2.3 and later. </p> 66 67 </ul> 68 69 <p> Although these approaches appear to be attractive, they have 70 some serious limitations that you need to be aware of. First, 71 content inspection software must finish in a limited amount of time; 72 if content inspection needs too much time then incoming mail 73 deliveries will time out. Second, content inspection software must 74 run in a limited amount of memory; if content inspection needs too 75 much memory then software will crash under a peak load. Before-queue 76 inspection limits the peak load that your system can handle, and 77 limits the sophistication of the content filter that you can use. 78 </p> 79 80 </dl> 81 82 <p> The more sophisticated content filtering software is not built 83 into Postfix for good reasons: writing an MTA requires different 84 skills than writing a SPAM or virus killer. Postfix encourages the 85 use of external filters and standard protocols because this allows 86 you to choose the best MTA and the best content inspection software 87 for your purpose. Information about external content inspection 88 software can be found on the Postfix website at https://www.postfix.org/, 89 and on the postfix-users (a] postfix.org mailing list. </p> 90 91 </body> 92 93 </html> 94
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