An image embedded in an email must be downloaded off a server by the recipient. This allows the sender to determine exactly
when an email message is opened. Images will often be added surreptitiously and can be transparent. The general concept is known as a web beacon or tracking pixel. It can be made to work not only with images but other elements of HTML.
The pixel will disclose the IP address to help pinpoint the recipient’s location.
Email software can be set to prevent the downloading of images from servers. In Outlook, go to File . . . Options . . . Outlook Options. In the Trust Center‘s ’Automatic Download’ tab check off the boxes labeled “Don’t download pictures automatically in standard HTML email messages or RSS items” and “Don’t download pictures in encrypted or signed HTML email messages.”