After you've created a new message, open the Format menu. The current message format is indicated with a check mark; to change it, choose the format you want to use (in this case, Plain Text).
If you have already typed the text for your message, Outlook notifies you that any formatting applied to your text will be lost; click Yes to continue.
Text in your message will be stripped of its formatting attributes; send the message as usual.
Not everyone uses the same mail program. As a result, some people may not be able to read e-mail messages you send to them if your message's format is not one that their program supports. An e-mail message's format dictates what types of formatting can be applied to that message.
Types of Formats
Outlook's default format, HTML, provides the widest range of formatting options, enabling you to make text bold or change its color, even to include graphics. Another option, rich text, allows for some formatting. The third choice, plain text, is what the name implies: plain text. You can change the format used for a mail message.