The format describes the encoding and format for the ezine. The format field is optional.

A plain text format is the most common - it means there is no special formatting, and the message is typically displayed by mail readers exactly how it was composed. It is text only, with no pictures.

HTML formatted ezines (also called "Rich Text Formatting" by Microsoft ® Outlook) can contain anything that can appear on a web page, including graphics, sound and video clips, and animations. It usually looks better, but it's a lot bigger.