Source: Roaring Penguin Software
The flow of email on the Internet is both surprisingly simple and surprisingly complex. It is simple because the basic email protocols are decades-old plain-text protocols with very simple definitions. It is complex because many tiny details can have a profound influence on how mail flows and on how to interpret message headers. This paper will teach how email flows on the Internet, and how to interpret message headers to determine the history of a piece of email. These are valuable skills for troubleshooting the email setup or even just for figuring out how a particular email arrived in one's inbox.