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<primary><emphasis role="pkg">postgrey</emphasis></primary>
“Greylisting” is a filtering technique according to which a message is initially rejected with a temporary error code, and only accepted after a further delivery attempt with some delay. This filtering is particularly efficient against spam sent by the many machines infected by worms and viruses, since this software rarely acts as a full SMTP agent (by checking the error code and retrying failed messages later), especially since many of the harvested addresses are really invalid and retrying would only mean losing time.
Postfix doesn't provide greylisting natively, but there is a feature by which the decision to accept or reject a given message can be delegated to an external program. The <emphasis role="pkg">postgrey</emphasis> package contains just such a program, designed to interface with this access policy delegation service.
<primary>Postfix</primary><secondary><literal>check_policy_service</literal></secondary>
Once <emphasis role="pkg">postgrey</emphasis> is installed, it runs as a daemon and listens on port 10023. Postfix can then be configured to use it, by adding the <literal>check_policy_service</literal> parameter as an extra restriction:

smtpd_recipient_restrictions =
permit_mynetworks,
[...]
check_policy_service inet:127.0.0.1:10023
Each time Postfix reaches this rule in the rule set, it will connect to the <command>postgrey</command> daemon and send it information concerning the relevant message. On its side, Postgrey considers the IP address/sender/recipient triplet and checks in its database whether that same triplet has been seen recently. If so, Postgrey replies that the message should be accepted; if not, the reply indicates that the message should be temporarily rejected, and the triplet gets recorded in the database.
The main disadvantage of greylisting is that legitimate messages get delayed, which is not always acceptable. It also increases the burden on servers that send many legitimate emails.
<emphasis>IN PRACTICE</emphasis> Shortcomings of greylisting
Theoretically, greylisting should only delay the first mail from a given sender to a given recipient, and the typical delay is in the order of minutes. Reality, however, can differ slightly. Some large ISPs use clusters of SMTP servers, and when a message is initially rejected, the server that retries the transmission may not be the same as the initial one. When that happens, the second server gets a temporary error message due to greylisting too, and so on; it may take several hours until transmission is attempted by a server that has already been involved, since SMTP servers usually increase the delay between retries at each failure.
As a consequence, the incoming IP address may vary in time even for a single sender. But it goes further: even the sender address can change. For instance, many mailing-list servers encode extra information in the sender address so as to be able to handle error messages (known as <emphasis>bounces</emphasis>). Each new message sent to a mailing-list may then need to go through greylisting, which means it has to be stored (temporarily) on the sender's server. For very large mailing-lists (with tens of thousands of subscribers), this can soon become a problem.

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  1. Debian Handbook
  2. 11_network-services
  3. English

smtpd_recipient_restrictions =
permit_mynetworks,
[...]
check_policy_service inet:127.0.0.1:10023
3 years ago
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Source string description
Checked-By: Ryuunosuke Ayanokouzi; Ref: $ man 5 postconf; Tag: L-CFMD;
Flags
ignore-same, read-only, xml-text
String age
3 years ago
Last updated
11 months ago
Source string age
3 years ago
Translation file
string 248