The structure and rendering of multipart emails

When marked as multipart/alternative, an email message can contain multiple versions of the same content which allows the email client to render the message in the most appropriate format for the user's device. In most cases, two versions are included: the plaintext version (text/plain) and HTML version (text/html). Other content types such as AMP (text/x-amp-html) are often used in conjunction with the plaintext and HTML versions.