What are MIME types?

MIME types (also called media types) identify the format of content in email messages. They follow a type/subtype structure and tell receiving software how to handle each piece.

Common MIME types in email:

text/plain: Plain text content, no formatting

text/html: HTML formatted content

multipart/alternative: Message with multiple versions (typically plain text and HTML)

multipart/mixed: Message with attachments

image/png, image/jpeg, image/gif: Embedded or attached images

application/pdf: PDF attachments

application/octet-stream: Generic binary file

MIME types appear in the Content-Type header:

Content-Type: text/html; charset=UTF-8

Content-Type: multipart/alternative; boundary="----=_Part_123"

Email clients use MIME types to decide how to display content: render HTML, show images, offer attachment downloads, or warn about suspicious file types.

Incorrect MIME types cause problems: an HTML email sent as text/plain displays raw code instead of formatted content. Attachments with wrong types may not open correctly.