From < Mastering Microsoft Exchange Server 2013 By: David Elfassy Publisher: Sybex Pub. Date: November 18, 2013 Web ISBN-10: 1-118-55683-6
From < Pro Exchange Server 2013 Administration By: Jaap Wesselius Publisher: Apress Pub. Date: December 19, 2013
Spfwizard.net
https://mxtoolbox.com/EmailHeaders.aspx
