define SMTP (the envelope), while STD 11 and define the message (header and body), formally referred to as the Internet Message Format. SMTP defines the trace information of a message, which is saved in the header using the following two fields: Received: when an SMTP server accepts a message it inserts this trace record at the top of the header (last to first). Return-Path: when the delivery SMTP server makes the final delivery of a message, it inserts this field at the top of the header. A mail user agent (MUA) knows the outgoing mail SMTP server from its configuration. A MTA (or a relay server) typically determines which server to connect to by looking up the MX (Mail eXchange) DNS resource record for each recipient's domain name. The path depicted below can be reconstructed on the ground of the trace header fields that each host adds to the top of the header when it receives the message: Return-Path: <author@example.com> Received: from D.example.org by E.example.org with SMTP; Tue, 05 Feb 2013 11:45:02 -0500 Received: from C.example.net by D.example.org with SMTP; Tue, 05 Feb 2013 11:45:02 -0500 Received: from B.example.com (b.example.com [192.0.2.1]) by C.example.net (which is me) with ESMTP id 936ADB8838C for <different-recipient@example.net>; Tue, 05 Feb 2013 08:44:50 -0800 (PST) Received: from A.example.com by B.example.com with SMTP; Tue, 05 Feb 2013 17:44:47 +0100 Received: from [192.0.2.27] by A.example.com with SMTP; Tue, 05 Feb 2013 17:44:42 +0100 It is important to realize that the first few lines at the top of the header are usually trusted by the recipient. In fact, those lines are written by machines in the recipient's Administrative Management Domain (ADMD), which act upon her or his explicit mandate. By contrast, the lines that prove the involvement of A and B, as well as of the purported author's MUA could be a counterfeit created by C. The Received: field shown above is an epoch-making piece of the header. The Return-Path: is written by E, the mail delivery agent (MDA), based on the message envelope. Additional trace fields, designed for email authentication, can populate the top of the header. Normally, messages sent out by an author's ADMD go directly to the destination's MX (that is B → D in the figures). The sender's ADMD can add authentication tokens only if the message goes through its boxes. The most common cases can be schematized as follows: Sending from within ADMD's network