After checking some spam I received, I noticed that spoofers seem to want to manipulate the Reply-To address, so perhaps the key to stopping spoofing is to stop the ability to change the reply-to address:
From Google Incorporation® noreply@googleuk.com
Reply-To portrugpat@outlook.com
Date 2017-05-24 7:42 am
Priority Normal
We wish to congratulate you on this note, for being part of our selected winners in our just concluded …