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

Why do you want to know that someone has read the mail?

Legally speaking, no way you might come up with will hold in court (say you want to make sure that someone has read an important document) because you can't prove that the reader was the recipient.

Security wise, your mail will be flagged by any decent spam filter as spam and deleted before anyone can see it because adding strange links or web bugs or images in a mail is just what the spammers do. And if you want to sell something, this is a surefire way to make sure you won't.

UPDATE: If you need to make sure people have read something, I suggest to send the mail with a link or a pointer to the document they should read. Don't add web bugs or personalized IDs to the link. In the document, tell them to sign the document and return a signed copy as proof that they have read it.