What is a Milter (mail filter)?

About

A milter or mail filter is a filter extension for SMTP server (mail transfer agents - MTA) that transforms a email message

Protocol

It refers to a protocol and API for communicating mail traffic information between:

• MTAs
• and mail filtering plug-in applications.

It was originally invented at Sendmail, Inc. but has also been adapted to other MTAs.

This protocol is used to inspect during a SMTP transaction:

• the TCP connection events (CONNECT, DISCONNECT),
• the SMTP commands (HELO, MAIL FROM, etc.)
• the email message (headers and body)

It's used by:

• Sendmail
• and Postfix

List

• DKIM
• SRS

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