"maintains a list of people who are allowed to send you mail. This list is called a whitelist. When somebody who is not on your whitelist sends you an email message, they get a very simple message in response ... When the sender gets this email message, all they have to do is click reply and then click send. Spamthing scans all of your incoming email for a message from the particular sender that has the words SPAMTHING #119285431 in the Subject: line. When it finds this message, it automatically adds the sender to your whitelist and sends them [a] polite message in response."