A mailer, who is owned by Alice, which sends mail which looks like spam to the source, who will be called Bob. It adds a random number, which is stored on Alice's machine, to the subject line and attaches the original text of the mail to the outbound mail. If Bob is actually a person, rather than a mail relay for example, and wants to get the mail through to Alice, he then uses the reply, then the send function on his mailer. When the mailer on Alice's machine sees the mail, if in the subject line it has a number equal to one of the numbers stored, it lets the mail through, reincorporates the text attachment into the message and lets it through to Alice.
 
 

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