Multi-part Formats
Many email servers are configured to automatically generate a plain text version of a message and send it along with the HTML version, to ensure that it can be read even by text-only email clients, using the Content-Type: multipart/alternative, as specified in RFC 1521. The message itself is of type multipart/alternative, and contains two parts, the first of type text/plain, which is read by text-only clients, and the second with text/html, which is read by HTML-capable clients. The plain text version may be missing important formatting information, however. (For example, an equation may lose a superscript and take on an entirely new meaning.)
Many mailing lists deliberately block HTML email, either stripping out the HTML part to just leave the plain text part or rejecting the entire message.
Read more about this topic: HTML Email