Sendmail 8.13.3 Programmer's Guide

Introduction
Milter Overview
Chapter 1 3
Milter Overview
Sendmail 8.13.3 contains an advanced and effective mail filtering facility
called Milter, which stands for Mail Filter. Milter is both a protocol and a
library. Milter APIs provide an interface for third-party software to
validate and modify messages as they pass through the mail transport
system. Milter APIs enable filters to “listen in” to the SMTP conversation
and modify Simple Mail Transfer Protocol (SMTP) responses.
To modify aspects of the message, you can call the Milter library
functions that send special messages to Sendmail. The Milter library is
multi-threaded and a given Sendmail installation can have multiple mail
filters. Sendmail is single threaded but it forks into multiple processes.
Sendmail uses mail filters to filter incoming SMTP messages.
Milter APIs provide the following benefits:
• Safety and security – You do not need root user privileges to run the
filter processes. This feature simplifies coding and limits the impact
of security flaws in the filter program.
• Reliability – Any failure in the Milter program does not affect
Sendmail. When the Milter program fails, Sendmail either considers
that the Milter program does not exist or considers that the required
resource is unavailable.
• Simplicity – You can use Milter APIs to easily implement filters in
Sendmail. To make the implementation easy, you can use threads by
defining thread-clean interfaces that include local data hooks.
• Performance – A simple Milter program does not degrade the
performance of Sendmail.
Following lists the types of Milter APIs:
• Library control functions
• Data access functions
• Message modification functions
• Other message handling functions
• Callbacks
Milter APIs operate in the following phases: