Viewing HTML email is a nasty way to get tracked by spammers. The spammers put images in with names coded to your email address and as soon as you view the image, the spammer knows you read the email and now your email is more valuable to other spammers!
Go to Preferences, choose Viewing and turn off the showing of HTML messages. However, with Panther’s Mail application, if you do get an HTML message that is legitimate (say an Apple newsletter) a nice “Load Images” button will appear for you to click. And now, mail will actually render the HTML text, just won’t load the images!