MIME extends Internet message format so a message can contain more than plain ASCII text. It defines media types, multipart structure, transfer encodings and related headers.

A common message might look conceptually like:

multipart/mixed
├── multipart/alternative
│   ├── text/plain
│   └── text/html
└── application/pdf

Why multipart matters

multipart/alternative carries different representations of the same content, often plain text and HTML. multipart/mixed commonly combines a body with attachments.

Boundaries separate the individual body parts.

Encoding is not encryption

Base64 and quoted-printable make content safe to transport through mail systems. They do not provide secrecy. Anyone with the message can decode them.

Reference material