D) DKIM, SPF and DMARC
Read this article for a general description.
SPF stands for Sender Policy Framework and is an authentication protocol that lists IP addresses in a DNS TXT record that are authorized to send email on behalf of domains.
DKIM stands for Domain Keys Identified Mail and acts as an ID or passport that can verify who you are. When you are sent from your email server, the server attaches DKIM so the receiving server can verify you. Technically speaking, DKIM authentication provides a method for validating a domain’s identity that is associated with a message through cryptographic authentication.
DMARC stands for Domain-based Message Authentication, Reporting & Conformance and is an email authentication, policy, and reporting protocol. It helps domains address domain spoofing and phishing attacks by preventing unauthorized use of the domain in the Friendly-From address of email messages.
This step is mandatory and critical for proper email delivery.
The three above pieces of technology are based on DNS records and keys and are able to clearly identify which mail has been sent by a genuine source and which is plagiarism and spam.
Installation
Simple emerge the following packages:
emerge -vp opendkim opendmarc mail-filter/spf-engine
Install spamassassin & amavisd-new