How Are Multipart Emails Structured?

How Are Multipart Emails Structured?

Multipart emails are structured using MIME (Multipurpose Internet Mail Extensions) format with multiple parts separated by MIME boundaries. Structure includes: MIME headers (Content-Type: multipart/alternative, multipart/mixed, etc.), MIME boundaries (unique strings separating parts), multiple parts (HTML part, plain text part, attachment parts), and part headers (Content-Type, Content-Disposition, Content-Transfer-Encoding for each part).

Multipart/alternative contains HTML and plain text alternatives. Multipart/mixed contains text and attachments. Multipart/related contains HTML and inline images. Email clients parse MIME boundaries to identify and display appropriate parts.

Learn more about MIME in email and analyze email source code using our email source analyzer.

Multipart Structure

MIME Headers

Multipart emails start with MIME headers specifying multipart content type and boundary:

• Content-Type: multipart/alternative; boundary="boundary-string"
• Content-Type: multipart/mixed; boundary="boundary-string"

MIME Boundaries

MIME boundaries separate parts. Boundaries are unique strings that mark the start and end of each part.

Part Structure

Each part contains: part headers (Content-Type, Content-Disposition, Content-Transfer-Encoding), blank line, and part content.

Part Order

Parts are ordered by importance. Email clients typically display the first compatible part.

Multipart Types

Different multipart types serve different purposes:

multipart/alternative

Contains alternative versions of the same content (HTML and plain text). Email clients choose which version to display based on capabilities.

multipart/mixed

Contains mixed content (text and attachments). All parts are displayed together.

multipart/related

Contains related content (HTML and inline images). Parts are related and displayed together.

multipart/signed

Contains signed content (message and signature). Used for email authentication (DKIM).

Nested Multipart

Multipart emails can be nested (multipart/mixed containing multipart/alternative) for complex email structures.

MIME Boundaries

What Are Boundaries?

MIME boundaries are unique strings that separate different parts in multipart emails. Boundaries mark the start and end of each part.

Boundary Format

Boundaries are specified in Content-Type headers: Content-Type: multipart/alternative; boundary="unique-boundary-string"

Boundary Usage

Boundaries appear before each part (except the first): --boundary-string and after the last part: --boundary-string--

Boundary Parsing

Email clients parse MIME boundaries to identify and extract different parts for display.

Unique Boundaries

Boundaries must be unique within each email to ensure proper parsing.

Email Client Handling

Part Selection

Email clients select which part to display based on client capabilities and user preferences (HTML vs plain text).

Part Parsing

Clients parse MIME boundaries to identify and extract different parts for display.

Part Display

Clients display appropriate parts: HTML content for clients with HTML support, plain text for clients without HTML support.

Attachment Handling

Clients extract and display attachments from multipart/mixed messages separately.

Error Handling

Clients handle missing or invalid parts by displaying available parts or falling back to plain text.