MIME
Multipurpose Internet Mail Extensions — the format that lets email carry attachments, HTML, and non-ASCII characters.
MIME (Multipurpose Internet Mail Extensions) is the foundational standard that extended early email beyond plain ASCII text to support attachments, HTML content, non-ASCII characters, and multipart messages. Every modern email is a MIME message; the format defines how the parts are structured and labeled.
A multipart MIME message contains multiple parts (typically a plain-text version and an HTML version of the same content) joined by a boundary string. Email clients pick which part to render. The structure also handles attachments (as separate base64-encoded parts) and inline images.
MIME issues usually manifest as rendering bugs: HTML showing as raw markup, attachments appearing as garbled inline content, character-encoding leaks (=XX patterns from quoted-printable). Most ESPs handle MIME correctly; custom sending infrastructure is where bugs creep in. When debugging client-render issues, inspect the raw MIME source before assuming the HTML is the problem.