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Originally Posted by Head Boy
You need to hit spam at source. It wouldn't take much to check the sending ip against the domain in the envelope - if they didn't match, then you dropped the mail item (or bounced it, that would be more effective  ). |
That's what SPF and Sender ID does, and it breaks down the moment someone sets up email forwarding (since the server forwarding the mail will have a different IP from the expected one for the domain).
DomainKeys / DKIM has a similar effect, but does it better in my opinion: the message body and some of the headers are signed by the originating mail server; the receiver can then check that the message has the proper signatures for a message coming from that domain. It doesn't matter if the email has been forwarded or sent through a mailing list, as long as the signature and signed content remains intact. And only those servers authorized by the domain owner (through DNS) will have the necessary private keys to do the signing.
Neither method tells you if the message is spam; it just tells you whether it really came from the domain it says it came from.