Have you ever wondered whether or not someone opened the email you sent using Gmail? You can use Mailtrack to do just that. Mailtrack is a Chrome plugin that adds a tracking pixel to your outgoing message. When your recipient opens your email, the image is requested and Mailtrack registers the open, indicating that it’s been seen with a checkmark in your Gmail interface.
This is such a simple, effective tool that I wish every email client would incorporate it into their platform. Virtually all emails are sent and seen in HTML format nowadays, so it’s an effective solution. If you’re customer is using Outlook, it often blocks and doesn’t down images by default so you may not register the open even if someone did.
Mailtrack has a simple interface to see what emails have been opened and read.
Gmail should just buy this app from these folks and integrate it into their platform along with an API methodology to use with clients. An email client outside of your Chrome Browser isn’t going to add the tracking pixel.