This repository contains scripts to prevent a mail server from generating
backscatter: https://en.wikipedia.org/wiki/Backscatter_(email)

Traditionally, preventing the generation of backscatter has required
either rejecting undeliverable messages during the SMTP transaction, or
discarding undeliverable messages.  The former is not always technically
feasible, and the latter is very bad for usability (since legitimate
senders receive no feedback that their message couldn't be delivered).

This repository proposes a better solution: bounce undeliverable messages
only when the return address can be authenticated using either SPF
or DKIM.  This eliminates backscatter, since bounces will never be sent
to a forged return address, while still providing feedback to users of
modern mail providers which support SPF or DKIM.

This repository is divided into two sub-directories:

milter - contains a sendmail-/postfix-compatible mail filter (milter)
that implements the above logic for bounces originated by your MTA.

mailman - contains a Mailman extension that implements the above logic for
bounces generated by Mailman (such as those generated when a non-member
posts to a members-only list).