greylist

Fast, ultra-customizeable, pluggable DNS server (forwarding, recursive, or none) with caching, filtering, routing, DNSSEC, admin UI, and plugins. UDP/TCP/DoT/DoH.

qmail fork with IPV6, TLS, DANE, DKIM, SRS2, SPF, daemontools, qmailanalog, mess822, & ucspi-tcp

The Mailismus mailserver - SMTP, IMAP, POP3, SSL

Web interface for SQLgrey built on Laravel 5

List of "big mailers" for OpenBSD's 'smtpctl spf walk' mechanism

gld, a lightweight greylist daemon for postfix

Postfix policy application written in Go

dynamic and optionally distributed greylisting daemon

Verify email(s) using correct SMTP conversations (HELO / MAIL FROM /RCPT TO)

OpenSMTPd filter for quick and dirty greylisting

A tool to kep the Unifi Country Blocklist Updated

Haraka plugin that implements greylisting