Messages and MIME
Important message fields
Use originator, destination, date, subject, identifier, and trace fields without treating them as the SMTP envelope.
8 minute lesson
From identifies the message author. Sender can identify the agent that transmitted it on an author’s behalf. To and Cc are displayed destinations; Bcc recipients should not appear in the delivered message.
Date is the message creation date. Message-ID is a globally unique identifier generated by the originating system. Received fields are added by transport systems and form a trace you normally read from bottom to top.
None of these replaces the SMTP envelope. The envelope controls the current delivery transaction.
Consider a mailing-list message:
From: Alice <alice@example.org>
Sender: Announcements <list@example.net>
To: Product list <product@example.net>
Message-ID: <launch-20260730@example.org>From names the author. Sender says another agent sent it. The envelope return path may point to a per-recipient bounce address that appears in neither field.
Message-ID helps conversations, logs, and duplicate investigation, but it is not an authorization token or guaranteed proof of authorship. Authentication systems such as DKIM evaluate other evidence.
Each receiving SMTP server prepends its own Received field. Start with the bottom field to see the earliest recorded hop, then move upward. Compare timestamps with their numeric time zones and allow for clock mistakes.
Treat displayed fields as untrusted input. A friendly display name can contain misleading text, and Reply-To can direct responses somewhere other than From.
Inspect a raw message and write a table with author, sender, reply destination, message identifier, envelope return path, and oldest recorded hop.
Lesson completed