Setting Netscape to Plain Text Mode

Netscape Mail 7.0, 7.1. 7.2

Compose your message as usual. Select "Options" / "Format" / "Plain Text Only from the message's menu. Continue editing the message, and finally send it using the "Send" button.

To set Netscape to always send in Plain Text Mode select "Edit", then "Mail & Newsgroup Account Settings..".  In the main "Account Settings" screen, uncheck the box "Compose Messages in HTML format".  Press the "OK" button.