Check if email has been uploaded with image embed or otherwise

Question

I’m sending an email notification 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 was to use an image like this.

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

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

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

I have some questions: has possibility to provide a confirmation? url or if not have. how I would read this return email and process it so that I could put a read flag on it?