Mac OS X Mail has an option for the security and privacy conscious email user that disables downloading of any content from the net.
Prevent Mac OS X Mail from Downloading Remote Images
To prevent Mac OS X Mail from downloading remote images:
- Select Mail | Preferences... from the Mac OS X Mail menu.
- Go to the Viewing category.
- Make sure Display remote images in HTML messages (or Display images and embedded objects in HTML messages, depending on the Mail version) is not selected.
- Close the preferences window.