Email message headers have a lot of information in them. Only the tiniest amount of that header information is typically exposed to a recipient by default. There's much more to it than just the from address, to address, subject line and date. There's routing information, authentication information, IP address information, and more information hiding just beneath the surface, in the "full headers" or "internet headers" of that email message. To get at the rest of that header information associated with a given email message, a recipient will need to "view full headers" or "view the message source."