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Email Structure and Headers

Email Format (RFC 5322)​

An email consists of:

  1. Headers - Metadata about the email
  2. Blank Line - Separator
  3. Body - The actual message content

Common Email Headers​

From: sender@example.com
To: recipient@example.com
Subject: Email Subject
Date: Mon, 7 Sep 2026 10:30:00 +0000
Message-ID: <unique-id@example.com>
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0

Important Headers​

  • From: Sender's email address
  • To: Primary recipient(s)
  • Cc: Carbon copy recipients
  • Bcc: Blind carbon copy (hidden recipients)
  • Subject: Email subject line
  • Date: When email was sent
  • Message-ID: Unique identifier for the email
  • Reply-To: Where replies should go
  • Return-Path: Where bounces should go
  • Received: Server trail (added by each server)

MIME (Multipurpose Internet Mail Extensions)​

MIME allows emails to include:

  • Multiple content types (text, HTML)
  • Attachments
  • Non-ASCII characters

MIME Headers​

MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="boundary-string"

Example Multi-part Email​

--boundary-string
Content-Type: text/plain

This is the plain text version.

--boundary-string
Content-Type: text/html

<html><body>This is the HTML version.</body></html>

--boundary-string
Content-Type: application/pdf; name="document.pdf"
Content-Disposition: attachment

[Binary data here]
--boundary-string--