Is there a way to determine whether an e-mail reaches its destination?

Question

I have a PHP script that sends out critical e-mails that needs to reach its destination. I know how to check whether the e-mail sent successfully, the only issue is knowing

Answer 1

The SMTP protocol is asynchronous, so the only guaranteed feedback can be returned by the users mail client. In case there are problems with the delivery you will get back bounce emails to the address defined in the envelope sender (mail header Return-Path) Timing of these bounces are unpredictable and undefined. Tracking pixels in the HTML email body are so far the best way to report you that the user has actually opened the email.