How does it work?
If a mail is sent with "MAIL FROM:" an email address and "RCPT TO:" email@example.com, a 550-response indicates that the email address is not subscribed to the list. A restful php service (source at check-subscr.php.txt) does the SMTP operation and returns JSON-object indicating if the address is subscribed to the lists. Attention: the script does not work if the domain of the supplied email address does not exist. The script does not work if the same address is checked very often for subscriptions, in this case the returned JSON array has value of -2.
Written in November 2008 by Дилян Палаузов