Emails currently can be delivered in plain-text format or HTML format. HTML allows the email to contatin images and for the text to be formatted, as you might a Word document. Plain text is delivered in Courier New font with no formatting.
When they don't know which format to use for a recipient, most senders opt for Multi Part. This is because in general they'd prefer the recipient received the HTML version and most email clients are set to accept HTML as a default.
