Check if e-mail was embedded with embedded image or other form

I am sending email notifications with phpmailer. These notifications are very important because they are firewall reports and I need to know when the person who should receive them read this message.

I researched and one of the ways I thought I was using an image in this way.

<img src='http://site.com/minhaimagem.php?id_envio=762'>

Only gmail and other webmails block this type of use. is there a way for me to know using phpmailer like this:

$mail->addCustomHeader("Disposition-Notification-To: [email protected]");

I have some doubts: Can you provide a confirmation url? or if in the case of not having. how would I read this return email and process it so I could by a read flag on it?