PixelBlock is a Gmail extension that blocks people from tracking when you open/read their emails.
PixelBlock is an Gmail extension that blocks email tracking attempts used to detect when you open/read emails. PixelBlock displays a 'red eye' when it finds and blocks a tracking attempt inside of an email you're reading.
Update (11/20/2015): The no images issue when printing or viewing an email in a popup window is now fixed
Note: Using PixelBlock will not interfere or prevent you from using any email tracking software on the outgoing emails you send. It just protects you from being tracked by others.