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Defense against spam
DKIM (DomainKeys Identified Mail) is a cryptographic method for signing mail. Mail is signed with the private key on the mail server. The public key may be retreived from the domains TXT record for verification.
The SMTP protocol was not originally designed to be very resistant to spoofing. SPF tries to help by reading rules about sender servers from the domain TXT records.
SPF does not take redirected mail into account. Due to SPF, all redirected mail would be discarted as fake.
Don’t use SPF. Instead, use DKIM.
DCC (Distributed Checksum Clearinghouses) creates a checksum from all mail. If the same checksum is detected many times, the spam score is increased.
Pyzor is an offspring of Razor and works in a similar manner. Mail checksums are collected and checked against multiple rules on central servers.
Razor is a closed source community spam filter. Further reading: Vipul’s Razor