This is a neat tip that I just discovered. I’m really happy that it works in both Snow Leopard and Lion. First the tip – Open Mail > Preferences and click on the Viewing Tab. Uncheck the box next to “Display remote images in HTML messages”.
Spammers will send out messages that contain graphic images that appear to be embedded in a message, but are actually linked from a Web site somewhere. Spammers use that embedded-graphics trick to discover that their message has gone to a live, active email address. Of course, if you never open the message that works too, but we all have opened spam messages at one time or another.