Milter is set of API that provides an interface for third-party software to validate and modify messages as they pass through the mail transport system.
Milter was portmanteau for mail filter. Originally milter was written for sendmail. In 2006, postfix 2.3.0 officially support milter, although there are little limitations of Postfix milter implementations.
A milter-capable MTA will notifies the milter application about each phase of the delivery of a message. At each phase of the SMTP session, the milter application is given data about the arriving message and then has an opportunity to terminate acceptance of the message early when appropriate.
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. The filters also 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.
Website www.milter.org provides catalog of open-source and commercial milter. This website also provides the documentation about milter API for developer.
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