Open rates are tracked with the use of a transparent one pixel gif image hosted on a server and inserted into a HTML message just like usual images. Any action on the recipients’ part that leads to the image load is counted as an open. But this metric may not be accurate if:
- the recipient prefers receiving plain text messages;
- the recipient open a HTML email in a non-HTML compatible email client;
- the recipient’s email client doesn’t load the images by default;
- the recipient opens the email offline after download.
As a result all the above "email opens" will not be counted in the stats.
The open rate is generally defined as "percentage of unique email opens from the total of emails delivered". People can open the same email several times, and some companies measure open rates based on total opens rather than unique opens that results in overstated open rates. Some marketers equate the "email opens" to the "email reads" that may not be true at all.
It is important that you clearly define how you will measure the email open rate for your company, and then consistently work at improving it (from 40% to 50%, for example) without paying attention to someone else’s 80%.