An Internet message is a set of header fields followed by a blank line and the message body.

From: Alice <alice@example.com>
To: Bob <bob@example.net>
Date: Fri, 3 Oct 2026 14:00:00 -0500
Message-ID: <example123@example.com>
Subject: Example

This is the body.

SMTP transports this message, but SMTP’s envelope is separate from these headers.

Header folding and parsing

Message syntax has detailed rules around field names, bodies, line length and historical forms. Software should use a standards-aware parser rather than splitting raw headers on colons and hoping for the best.

Reference material