Email Headers & MIME

After DATA, the client sends:

From: Alice <[email protected]>
To: [email protected]
Subject: Hello
Date: Mon, 13 May 2024 10:30:00 +0000
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8

Body text...

Headers + blank line + body. RFC 5322 defines the format.

Multi-part messages use MIME (RFC 2045-2049):

From: [email protected]
To: [email protected]
Subject: Multi-part
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="----abc123"

------abc123
Content-Type: text/plain

Plain text version
------abc123
Content-Type: text/html

<p>HTML version</p>
------abc123
Content-Type: image/png; name=photo.png
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=photo.png

iVBORw0KGgoAAAANSUhEUgAA...
------abc123--

Boundary string separates parts. Each part has its own headers.

Common header types:

• Mailbox: From, To, Cc, Bcc, Reply-To. Comma-separated lists.
• Identification: Message-ID, In-Reply-To, References (for threading).
• Trace: Received (one per hop). Multiple, top-to-bottom most-recent first.
• Resent-*: forwarded mail trail.

Encoding:

• ASCII-only originally.
• Non-ASCII via Quoted-Printable or Base64 in body (Content-Transfer-Encoding header).
• Subject with non-ASCII: encoded via RFC 2047 (=?utf-8?B?...?=).

Threading via Message-ID + References + In-Reply-To: clients build conversation trees.

Bounce messages: Delivery Status Notifications (DSN, RFC 3464). Returned to MAIL FROM address with structured failure info.