When you send out an email campaign, one of the key metrics to measure the performance is the open rate. But do you know how email marketing programs tell if an email is opened?
The answer is a tiny, invisible image. When sending out emails, email marketing programs automatically insert a tracking pixel, which is an image of size 1×1 pixel, and invisible. When the recipient opens the email, the pixel is loaded from the server, and the email marketing programs know who opened the email and when.
While this method is very successful and is used by all major email marketing programs, there are two shortcomings:
1. If a recipient does not enable image display, then the tracking pixel won’t be loaded even when the email is opened. We did a study and found the open rate is about 40% under estimated.
2. The tracking pixel may be loaded by the recipient’s email server. The recipients’ email servers may scan the incoming emails for security purpose. In such cases, the email is not opened by the real recipient but is still considered as “open” the email marketing programs.