1. DKIM.org: Source: http://www.dkim.org/info/dkim-faq.html
  2. Wikipedia: http://en.wikipedia.org/wiki/DKIM
  3. Port25 Solutions, Inc.: Port25-Whitepaper-Authentication.pdf
  4. Sender Policy Framework: http://www.openspf.org/FAQ/How_does_it_work, http://www.openspf.org/SPF_vs_Sender_ID