Message formats
Microsoft Outlook can send and receive messages in three formats: HTML, Outlook Rich Text, and plain text. You can specify one of these three as a default message format that you want to use for most messages, but you can always switch to a different format for an individual message. The message format that you specify is independent of your choice of message editor. For example, you can choose Microsoft Word as your default message editor and then specify a message format to use in Word.
HTML
Choose HTML if you want these features: text formatting, numbering, bullets, alignment, horizontal lines, backgrounds, HTML styles, and Web pages. You can use Stationery and Signatures with HTML, but if you’re using Word 2000 as your e-mail editor, you must specify those options in Word. HTML is not available if you're using Word 97 as your e-mail editor.
Choose a default message format
On the Tools menu, click Options, and then click the Mail Format tab.
In the Send in this message format list, click the format you want.
Tips
If you use HTML as your new message format, you can use stationery to change the background and fonts used in the message.
By default, when you respond to a message in Microsoft Outlook, your reply is formatted the same way as the original message. For example, if you reply to a message sent to you in plain text, Outlook sends the response in plain text. However, you can change the message format of your reply. Place the insertion point in the message body, click the Format menu, and then click the name of a message format.