Milter (

portmanteau In linguistics, a blend—also known as a blend word, lexical blend, or portmanteau—is a word formed by combining the meanings, and parts of the sounds, of two or more words together.

for ''mail filter'') is an extension to the widely used open source

mail transfer agent Within the Internet email system, a message transfer agent (MTA), mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using the Simple Mail Transfer Protocol. In some contexts, the a ...

s (MTA) Sendmail and Postfix. It allows administrators to add mail filters for filtering

spam Spam most often refers to: * Spam (food), a consumer brand product of canned processed pork of the Hormel Foods Corporation * Spamming, unsolicited or undesired electronic messages ** Email spam, unsolicited, undesired, or illegal email messages ...

viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almo ...

in the mail-processing chain. In the language of the art, "milter" refers to the protocol and API implementing the service, while "a milter" has come to refer to a filter application that uses milter to provide service.

History

Prior to the advent of milter, an email filter was generally implemented as a program to which an MTA would hand the message once it has completely arrived, with most of the message's envelope information removed. That program could then analyze the header and body of the message and make a decision to accept the message (i.e. return a "success" status to the MTA) or reject it (i.e. return a "failed" status to the MTA). The MTA would then log a successful delivery or return a failure message to the sender as appropriate, and the filter would be responsible for affecting the delivery of the message (to the intended inbox(es) as-is, or modified to remove unwanted content, or to specific folder(s), etc.). An MTA that is milter-capable instead notifies filters to which it is connected about each phase of the delivery of a message, from initial client connection through completion of transmission. At each phase of the

SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typi ...

session, the filter is given data about the arriving message and then has an opportunity to terminate acceptance of the message early when appropriate. For very large messages, this can have an enormous impact when a decision to reject can be made as early as possible. Moreover, unlike the former model, a milter-capable MTA can connect to multiple filters in parallel that serve specific purposes such as anti-virus, anti-spam, message authentication, flow regulation, etc. Finally, such filters can take special action on the message: add or remove recipients in the envelope; alter the body prior to delivery; add, change or remove header fields in the message, etc. The Sendmail Consortium includes a special thread-based library in the sendmail distribution that provides the milter

API An application programming interface (API) is a connection between computers or between computer programs. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how to build ...

MTAs

Sendmail

Milter was first officially introduced to Sendmail 8.12.0 released on September 8, 2001. Milter was available in sendmail 8.11.x versions as an FFR (For Future Release/Beta code). Milter has undergone substantial improvements since its first official release.

Postfix

Milter support was added to Postfix 2.3.0 released July 12, 2006. Many, but not all, milters can be used with Postfix "out of the box".

Interface

The Sendmail Consortium provides a documented

Application Programming Interface An application programming interface (API) is a connection between computers or between computer programs. It is a type of software Interface (computing), interface, offering a service to other pieces of software. A document or standard that des ...

that can be used to create milters for special needs. The library provided by The Sendmail Consortium is pthread-based. However, documentation of the protocol used for communication between sendmail and milter processes is not provided. This internal protocol is subject to changes in new sendmail versions.

Sample Milters

* MIMEDefang * Amavis

References

{{Authority control Message transfer agents