About plain text messages
Plain text format is one that all e-mail programs understand; however, it doesn't support bold, italic, colored fonts, or other text formatting, and it doesn't support XML tags. It also doesn't support pictures displayed directly in the message body (although you can include them as attachments).
In most cases, you don't need to change the message format. When you send an HTML or rich text format message to others whose mail program doesn't understand these formats, their mail program automatically displays a plain text version in the message body. In some cases, you may prefer to use a different format for a specific message. For example, someone may send you a plain text message, but you may want to forward or reply to it using HTML or rich text formats and the features they support.