特殊:Badtitle/NS100:PostfixGreylisting

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Introduction

Greylisting is a spam-reduction technique which can be very effective. It works by delaying mail from untrusted senders. If the sending mail server is standards-compliant, it will re-send the e-mail, and the server will accept it. Most spam mailers, however, don't re-send the mail, and so the spam is blocked. Servers that re-send the mail will be added to a white list, and will not be delayed in future. This means that the first e-mail from a given sender will be delayed, but subsequent ones will not be.

Installation

Simply install the postgrey package and any dependencies. Note that it is in the universe repository. See the InstallingSoftware page for more information on installing packages.

Configuration

Run the following command to configure Postfix to use Postgrey:

sudo postconf -e 'smtpd_recipient_restrictions = permit_mynetworks,permit_sasl_authenticated,reject_unauth_destination,check_policy_service inet:127.0.0.1:60000'

Then run the following to reload the Postfix configuration:

sudo /etc/init.d/postfix reload

In Use

Once Postgrey is running, entries will start to appear in /var/log/mail.log. To see them, run sudo grep -i greylisted /var/log/mail.log. Entries will look something like:

Sep 14 10:44:57 mailserver postfix/smtpd[17049]: NOQUEUE: reject: RCPT from mail.server.com[1.2.3.4]: 450 <[email protected]>: Recipient address rejected: Greylisted for 300 seconds (see http://isg.ee.ethz.ch/tools/postgrey/help/somedomain.com.html); from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<mail.server.com>

In addition, e-mails that have been greylisted will have an X-Greylist header, eg:

X-Greylist: delayed 1201 seconds by postgrey-1.24 at mail.server.com; Fri, 14 Sep 2007 11:04:58 BST

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