When you create a message in Outlook, the default format is HTML. This format supports text formatting, numbering, bullets, alignment, horizontal lines, pictures (including backgrounds), HTML styles, stationery, signatures, and linking to Web pages. Because the most popular e-mail programs use HTML, it is the recommended format for Internet mail. It is also the recommended format if you send most of your messages within a company that uses Microsoft Exchange Server.

Plain text format

Plain text format is one that all e-mail programs understand. You can set Outlook to open messages you receive in plain text format only. However, plain text doesn't support bold, italic, colored fonts, or other text formatting. It also doesn't support pictures displayed directly in the message body (although you can include them as attachments).

Rich Text format

Outlook Rich Text Format (RTF) is a Microsoft format that only the following e-mail programs understand: Outlook 2003, Outlook 2002, Outlook 2000, Outlook 98, and Outlook 97. You can use RTF when sending messages within a company that uses Microsoft Exchange Server; however, it is recommended that you use HTML. RTF supports text formatting, including bullets, alignment, and linked objects. Outlook automatically converts RTF messages to HTML when you send a message to an Internet recipient, so message formatting is maintained and attachments will be received properly.