MIME
Multipurpose Internet Mail Extensions — the standard that defines how email messages encode non-ASCII text, HTML bodies, attachments, and other binary content within the plain-text structure of email.
MIME (Multipurpose Internet Mail Extensions, RFC 2045–2049) extends the basic RFC 5322 message format to support content beyond 7-bit ASCII text. Before MIME, email was limited to plain text in English; MIME introduced a system of Content-Type headers and encoding schemes that allow a single message to carry formatted HTML, images, PDF attachments, and text in any language.
The core mechanism is the Content-Type header, which declares the media type of a message part — for example, text/plain, text/html, image/jpeg, or application/pdf. A multipart message uses a boundary string to separate its parts, each with its own Content-Type and Content-Transfer-Encoding. Binary content is encoded with Base64 or quoted-printable so it can travel safely through text-based mail infrastructure.
Correctly parsing MIME is essential for any email reader. Mbox Viewer implements a MIME decoder that handles nested multipart messages, multiple encodings, charset declarations, and inline images. This decoding runs as part of the streaming parser so that even very large MBOX files are processed efficiently.
Related terms
A MIME message structure that combines multiple content parts — such as plain text, HTML, and attachments — in a single message, each separated by a unique boundary string.
The character encoding that specifies how bytes in a text part are mapped to readable characters. Common charsets include UTF-8, ISO-8859-1, and Shift_JIS; a mismatch causes garbled text known as mojibake.
A file — such as a PDF, image, or spreadsheet — embedded in an email message and encoded as a MIME part, separate from the message body, intended for the recipient to save or open.