Email forwarding is the process that occurs after a subscriber receives an email and selects the Forward option within their email client in order to send the email to another recipient.
The header of this forwarded email indicates the email came directly from the subscriber who originally received the email, through their email service. This type of forwarding is different from Forward to a Friend (FTAF), where the email appears to be coming from the original subscriber but the header indicates the email was actually sent from the original sender's email platform.