The Security Mentor

Spammers can use technical tricks to tell when you've opened email from them: see this InternetWeek article.

Spammers couldn't do this back when everyone got email as just plain text. Today a lot of mail programs display mail like it was a little Web page and that opens up lots of chances for mischief. In particular spammers can put a picture in the email that gets fetched from their computer when you look at it. Then their computer records who fetched it and when.

Outlook 2003 prevents this by blocking images from showing up in your email. Most non-Microsoft email programs also let you protect yourself.