I saw some people saying their newsletter has X open-rate. How can I know this thing? 
How do I know my email open rate?
email;marketing
The technique used to work out if an email has been opened or not is to put a tracker image with a unique identifier  in the request URL so your server can log that the email has been at least viewed. This image is normally just a 1x1px square that is invisible to the end user. The src of the image will be something like tracker.php?user_id=1312, then on your server, tracker.php takes that user_id string and can mark that user as opening that email.However, any figure you gain from from technique can't be taken expclicitly, there are a number of factors that can lead to both an over-estimate and under-underestimate:Most email clients ask you to download images purely for this reasonUsers may open the email but not read itUsers may not receive the HTML versionImages may be pre-fetched by virus scanners and scanned