73 lines
2.4 KiB
HTML
73 lines
2.4 KiB
HTML
<html>
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<head><title>Greylisting on SDF</title></head>
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<body>
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<h1>Greylisting on SDF...</h1>
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<h3>Definitions</h3>
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<menu>
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<li><strong>tuple:</strong>
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a single record combination of sender_IP(SMTP client),
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envelope_from(MAIL FROM:), and envelope_to(RCPT TO:).
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</menu>
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<h3>Described</h3>
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In short, it works like this. All incoming mail delivery attempts are told
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to "try again later; can't take delivery now". That in itself is not new,
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and all email transport systems know to expect those responses any time from any
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place, for any number of different reasons.
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<p>
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The positive benefit of Greylisting is that spam mail might not bother with a second delivery
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attempt.
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<p>
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Legitimate mail will try again, after a delay determined by them, they will (or should) attempt
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delivery again. When they do, if it is after the greylist period set by SDF (3 minutes at this writing), SDF
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will accept the message and whitelist the tuple (not indefinitely) so that
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the same tuple won't be delayed on near-future attempts.
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<p>
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MTA IPs may also be whitelisted at the system level
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<p>
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<h3>Do I want to use it?</h3>
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If your email address is not receiving very much spam, greylisting has no
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opportunity to be useful. Recommended practice is to leave it disabled until
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you want it because you receive too much spam.
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<p>
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<h3>SDF specific implementation</h3>
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Type/run unquoted "greylist --help"
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<br>results shown are not instantaneous, but updated every 2+ minutes or so.
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<br>greylist -gv will show the earliest time when the message will be accepted;
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and the three fields of the tuple.
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<br>greylist -wv will show the time when whitelisted entries will expire.
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<p>
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For VPM and VHOST accounts, you can use `mkvpm gry <user@domain>' to
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toggle greylisting on and off.
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<p>
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When the very first delivery is attempted, SDF issues a response ...
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<br>(host mx.sdf.org[192.94.73.18] said: 451 4.7.1 Connection deferred. (in reply to RCPT TO
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command))
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<p>
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Initial period during which a second delivery attempt will also be rejected: 3 minutes.
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<p>
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When the message is received by SDF, a header line (X-Greylist:) will be added which notes the
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delay time since first delivery attempt.
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<p>
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<h3>Additional Information:</h3>
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<a href="http://www.greylisting.org/">http://www.greylisting.org/</a>
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<br><a href="http://sdf.lonestar.org/index.cgi?faq?EMAIL?07">SDF FAQ EMAIL 07</a>
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<p>
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<hr>
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<cite>$Id: greylisting.html,v 1.9 2012/05/07 04:42:45 smj Exp $ </cite>
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</body>
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</html>
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