Multipart MIME Types in Email — mixed, alternative, related

Multipart Types in Email

Email messages use multipart MIME types to combine different content representations and attachments in a single message. Understanding these types is essential for anyone building email templates or parsing email bodies.

multipart/mixed

The most common email container type. It combines different types of content in sequence:

multipart/mixed
├─ text/plain (message body)
└─ application/pdf (attachment)

Use multipart/mixed when the email has attachments.

multipart/alternative

Contains alternative versions of the same content. The email client picks the best one it can display:

multipart/alternative
├─ text/plain (fallback)
└─ text/html (rich version)

Parts are ordered from simplest to most complex. The client typically displays the last format it supports.

multipart/related

Groups content where parts reference each other, such as an HTML body with inline images:

multipart/related
├─ text/html (references cid:logo)
└─ image/png (Content-ID: <logo>)

Real-World Email Structure

A typical rich email with attachments combines all three:

multipart/mixed
├─ multipart/alternative
│   ├─ text/plain
│   └─ multipart/related
│       ├─ text/html
│       └─ image/png (inline)
└─ application/pdf (attachment)

Quick Reference

Type	Use When
multipart/mixed	Message has attachments
multipart/alternative	Message has plain + HTML versions
multipart/related	HTML references inline images