Whitepaper entitled “Secure SMTP Proxy for Protecting Mail Transfer Agents”. This is a 70 page document that discusses new angles for defending mail daemons and more.
You can download it from the following link: https://packetstormsecurity.com/files/download/59101/secure_smtp_proxy_for_protecting_mta.tar.gz
Source: https://packetstormsecurity.com/files/59101/secure_smtp_proxy_for_protecting_mta.tar.gz.html