My scheme for improving smtp and reducing abusive spams that contain fake
headers basically amounts to a per domain blacklist implemented in DNS.
The mail server receiving the message will contact the DNS server of the "MAIL
FROM" domain with a lookup of (reversed IP address).example.com
The DNS server will respond with OK or DENIED or nothing based on it's
configuration. Specific entries allow an IP or set of IPs and a wildcard
entry disallows the rest of the internet.
OK means "Yes, send the message"
DENIED means "No, that IP address is not authorized to send mail from my
domain"
nothing means "timeout or admin has not configured their dns to support this
so send the message"
This method is completely backwards compatible,
useful when as little as 2 admins configure support,
capable of stopping klez type messages.
I have a further writeup at
http://www.vorteon.com/papers/spam_reduction_through_dns.html
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