It shows the return path of the message, i.e., the address that bounces will be sent to. When a message is stored in a mailbox, the return path is stored in a Return-Path field. Some mailers, in violation of RFC 1123, fail to record the return path of a message. Subsequent bounces are sent to an invented return path, typically the header From address. This behavior is a disaster for mailing-list messages, where the correct return path is the mailing list manager, and the From address is an innocent subscriber who has no way to remove bad addresses from the mailing list.
Every mailer is responsible for using the correct return path.
Received In theory, the value of a Received field is tokenizable . In practice, SMTP servers put all sorts of badly formatted information into Received lines. It is probably best for readers to treat everything before the final semicolon as unstructured text, purely for human consumption. A few SMTP servers don't even format the date correctly. Invalid Received fields are also produced by the cucipop POP server.