Who is an email really from? If an email client shows firstname.lastname@example.org as the sender most people would accept that at face value, but anti-spam filters and email clients are doing much more than just looking at the “from” address of an email. A single email can be relayed through several systems and computers on its journey from sender to recipient. After passing through multiple hands, who really sent the message? Increasingly, anti-spam filters and email service providers (ESPs) care about tracking this trail of servers to make sure that emails only passed through trusted hands.
Not much detail, but a nice quick primer.