We've done some internal testing and found out that only campaign emails where people click on a link are correctly tracked as "open". I can see the tracking pixel in the source of the received email at the very bottom. The email client we have used is latest Thunderbird and all images are displayed in the preview as well as when you open the mail (by double clicking). However for these actions, NO "opens" tracking happens. 

So this leaves us with very inaccurate "opens" statistics and needs addressing from Zoho.