Documents related to 'spam'
2006, RFC4406: Sender ID: Authenticating E-Mail
            2007, RFC4871: DomainKeys Identified Mail (DKIM) Signatures
            
            
            2009, RFC5429: Sieve Email Filtering: Reject and Extended Reject Extensions
            
            2010, RFC5782: DNS Blacklists and Whitelists
            2012, RFC6647: Email Greylisting: An Applicability Statement for SMTP