When an email is sent, it should contain no “Received” header fields. Each time the email is processed by a mail server along the delivery route, a new Received header field is prepended to the email. The very first Received header is added by the SMTP server that the SMTP client (such as your app or a program such as Outlook) connects to to send email.

A typical scenario is that an email will contain two Received headers when delivery is complete. The 1st is the Receive header added by your SMTP server. This may often be something like “localhost” and may show an internal IP address, such as “192.168.1.###”. Your SMTP server then relays the email to its destination, and the mail server at the destination (at the receiver) then adds its Received header.

The Received header has a syntax such as this:

        Received: from ? by ? via ? with ? id ? for ? ; date-time

Some mail servers may not include all the parts, and spammers often fake Received headers.