I am creating a tracker by PHP to get percent of opened email.
My code worked well if user open email by browser , but if user open email by an application (Mailbox for example) I can't detect it.
I don't know why but when I tried to saw log file, I saw that browser can requested to my server but native app can't.
<img src="Domain.tracker" width="1" height="1" style="display: none"/>
ANSWER:
When user open email by Application, you have to provide protocol to access your track server
<img src="http://Domain.tracker" width="1" height="1" style="display: none"/>