Currently there are a number of companies offering a service to embed zero pixel gifs in your outgoing emails, which make calls to specific servers with specific file locations that are stored in a database attached to the name of the recipient and details of the message. When the recipient opens the email with the embedded gif, it makes a request to earlier mentioned server, which exposes the user agent, location, and IP, of the person reading the message. It also tells the sender that the message was open, when it was opened, how many times, and from where.

The Internet is built on surveillance.