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SpamAssassin by Alistair McDonald

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How Spammers Verify Email Addresses

Spammers can verify email addresses in three ways: by receiving replies, providing an opt-out web link, and by using web bugs. Web bugs are small images that secretly track information about the spam recipient. The following section describes a web bug in detail. A recipient of spam may occasionally reply to a spammer's email. This is a futile gesture. Often, the spam email is sent from a fictional email address, and the reply will be returned, undelivered. Spam can also be sent from a real, but hijacked address. Hijacking occurs when the spammer chooses a bona-fide email address to use in the mail headers of his or her spam, either the From: header or the Reply-To: header. The unfortunate victim will receive ...

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