When an email containing certain kinds of image content is shown, users may see the banner "External content has been blocked to prevent the sender from identifying your personal information". This banner is inserted above the message:

The external content is most likely an image tag. These are frequently used for graphics and logos sent from centralized email systems. They are also used by spammers and there are documented methods for using specially crafted image tags to acquire browsing history information, so there is a security-issue potential.

What to do:

You can click on the 'Show Content' link to display the images for that email. You may also go to 'Preferences' and choose "Show external content immediately" to have all content downloaded as soon as they open any message. It is suggested you not enable the preference to automatically download external content immediately.

More info:

The default in Webmail is "Block external content until requested" for a good reason. Image tags and the like are used by people who want to know if someone has opened their message and may be used to deduce other information such as:

• Confirming valid email addresses;
• The type of computer and operating system that opened the message;
• Sites the user visited with that web browser.

The best practice is not to automatically download HTML graphics in email, whether by webmail or using a desktop email client.