PHP: Check who had read sent email?

Question

I am sending email to some users and wants to know who had read it, means if some one had read that email then a log file will maintain which contain the email address of th

Answer 1

Send a beacon image in the emails like so

And then use the beacon.php file to log the data. You will then want to output a 1X1 image with appropriate headers.

Important note
Many popular email clients (such as Gmail) now block external images, so this is by far, not fool proof.