Invisible pixels inserted into the content of an email and used to track activity are raising privacy concerns, ZDNet reports. The pixel automatically downloads when an email is opened, tracking when the email is read, the number of times it is opened, a user’s IP address and device usage. Hey messaging service Co-Founder David Heinemeier Hansson, who said the company processes 1 million emails with more than 600,000 blocked pixel tracker attempts daily, called them a “grotesque invasion of privacy.”