 <p>Presuming your website uses a "local" mail server, such as Postfix/Sendmail/Exim, you'd check the mail log. Generally (on Unix-ish systems), that's /var/log/maillog. It'll contain the full transcript of the email's passage through the mail server, from the moment it was handed off from PHP to when it goes out the door to the recipient.</p> <p>However, some accepting servers will LIE to you about the mail - they'll accept it with the usual "200 OK" success code, then just dump it in the trash because it triggered a spam filter or your IP/IP-block has been blacklisted somewhere.</p> <p>You could embed a web bug in the mail so that (hopefully) the client's email program will ping your server whenever they read the mail, but most mail clients now block remote images by default, so this is very unreliable as well.</p> <p>Basically, email is a best-effort-not-guaranteed-to-succeed business, and the entire system is set up to lie to you about everything.</p>