These links may help you distinguish pork shoulder from the other stuff.
"This document is intended to provide a comprehensive introduction to the behavior of email headers.
It is primarily intended to help victims of unsolicited email ("email spam") attempting to determine the real source of the (generally forged) email that plagues them; it should also help in attempts to understand any other forged email.
It may also be beneficial to readers interested in a general-purpose introduction to mail transfer on the Internet. "
"The ability to read and decipher email headers is a useful skill to learn for tracing messages to their original source and diagnosing many other problems. Headers may contain a lot of information but the most important information will always be contained in every email header."