General Guidance on combating spoofing
As an Escalation Engineer for Exchange Online we get lots of questions on how to stop email spoofing. It is a very broad topic and there are a number of things that can be done. Below is sone general guidance I provide to my customers when this topic comes up.
Read more at source: http://blogs.technet.com/b/circularlogging/archive/2016/01/06/general-guidance-on-combating-spoofing.aspx