In the analytics reports, email marketers will see a term called "Adjusted Open Rate" (AOR) under the Open Tracking tab. Sometimes the figure is the same as Open Rate, but sometimes it is larger. Why? We need to go back to the mechanism of open rate checking in order to understand the meaning of AOR.
To track open rate, 1and1Mail automatically insert a unique image pixel into the bottom of your HTML email. When your recipients open the email, 1and1Mail image server can capture the image pixel is loaded and mark the corresponding email as "Read".
But some email clients (e.g. Gmail, Yahoo Mail), by default, do not load the image in the HTML email unless the email is from a known person in the contact list. Such behavior of email clients is to protect the users because some images do contain virus or consume too much bandwidth in 3G/4G environment. In this case, although your recipients have opened your email, 1and1Mail server cannot track the action.
This checking mechanism is used by all mass mailers, so it is a universal problem. The actual open rate should be higher than the figure.
1and1Mail introduces Adjusted Open Rate (AOR) to make the data of Open Rate more accurate. AOR is a combination of both Open Rate and Click Rate. If your recipients open your HTML email without loading the image pixel, the open checking cannot be done, but if they click on any of the links in the email, 1and1Mail server will capture the click action and still mark the email as "Read".
Therefore, if you need to report any figures to your boss or record the data for future reference, Adjusted Open Rate is a more accurate figure that you should take.