Using signature headers to determine if an email has been forged
Email signatures, designed for authentication, non-repudiation, and spam control, can also be used to determine if an email has been forged. In the most common case, the forger copies headers from an existing email message to a new one. He could also attempt to change the content of a signed message. Regardless, the signature no longer corresponds to the message and it can be shown that the message is not authentic.