Milter

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A milter (portmanteau for mail filter) is an extension to the widely used open source mail transfer agents (MTA) Sendmail and Postfix. It allows administrators to add mail filters (for filtering spam, viruses...) very efficiently in the mail-processing chain.

Milter filters can reject a message during the SMTP session. Without milters, a spam filter has two options for dealing with mail it has classified as spam: it can generate a bounce message to the envelope sender of the message, in case its classification was incorrect, or it can fail to do so.

The envelope sender of spam messages is usually forged, so if the spam filter generates the bounce message, the spam filter runs a large risk of unintentionally forwarding the spam to an innocent victim, a phenomenon known as "backscatter spam". Backscatter spam both adds to the spam problem and creates a risk that the machine on which the spam filter is running will be classified as a spam source.

On the other hand, if the spam filter generates no bounce message, then legitimate senders whose email is incorrectly classified as spam will have no way to know that their message has been lost. Sometimes the sender can call the recipient on the phone to ask if they got the message, but that is not always practical.

Milters, on the other hand, preserve reliable email delivery without taking over responsibility for generating bounce messages. If the milter rejects a message, then the recipient's SMTP server never accepts responsibility for the message, and the sender's SMTP client will generate a bounce message, presumably to the real sender.

Sendmail.org includes a special thread-based library in the sendmail distribution which provides the milter API. The Sendmail daemon communicates with a milter process using the milter protocol.

The usage of milters has spread in recent years and milters are becoming increasingly important in mail processing on the large scale to block, verify and sort incoming mail.[citation needed]

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Please improve this article if you can (April 2008).

Contents

[edit] MTA

[edit] Sendmail

Milter was first officially introduced to Sendmail 8.12.0 released in September 8, 2001. Milter was available in sendmail 8.11.x versions as an FFR (For Future Release/Beta) code.

Milter has undergone impressive improvements since its first official release.

Search for MILTER in SENDMAIL RELEASE NOTES to know about changes related to Milter in sendmail versions [1].

Sendmail 8's successor, known as Meta1, will also support milters.[citation needed]

[edit] Postfix

Milter support was added to Postfix 2.3.0 released July 12, 2006 [2]. Not all milters can be used with Postfix out of the box, but many can[citation needed].

[edit] Postoffice SMTP server

Milter support since release 1.3

[edit] Interface

Sendmail provides a documented Application Programming Interface that can be used to create milters for special needs. The library provided by sendmail.org is thread based.

Sendmail.org does not provide documentation of the protocol used for communication between sendmail and milter processes. This internal protocol is subject to changes in new sendmail versions.

[edit] Sample Milters

[edit] External links

[edit] References

  1. ^ SENDMAIL RELEASE NOTES
  2. ^ Postfix before-queue Milter support