It refers to a protocol and API for communicating mail traffic information between:
- 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.)
It's used by:
- and Postfix