SPF Overview

SPF and Conventional Antispam Methods

DNS blacklists or blocklists (DNSBLs): IPv4 space is 32 bits wide; 2³² is about 4.2 billion—4.2 billion grains of sand would just about fill a pickup truck. Imagine trying to paint each individual grain black or white. IP-based blacklists are a valiant effort, but they operate at too low a level. A good DNSBL has to decide whether an IP address is spammy and get it right for each of the 4.2 billion IP addresses. No wonder DNSBLs come and go—their maintainers burn out and give up.

Right-hand side blacklists: RHSBLs use domain names, whereas DNSBLs use IP addresses. Domain names are a much better way to identify entities on the Internet, but RHSBLs haven't been quite as popular as DNSBLs. Why not? Spam doesn't come from spammer.net. It's forged from yahoo.com. That's why SPF helps: if spammers send mail with their true names, blocking them becomes trivial.

Address verification: at MAIL phase, you can check the validity of the envelope sender by attempting to send a test message to it. If the test comes back user unknown, you might not want to accept the message. This is useful because spammers often make up addresses at random. But as address verification becomes more common, spammers can be expected to forge actual addresses—all the more reason to use SPF.

Signature solutions: PGP/GPG and S/MIME users sign their messages. Recipients can check signature validity by downloading keys from a key server. Other schemes have been proposed in which the DNS itself acts as the repository for public keys. These solutions are good because .forward files continue to work without modification. They are bad, however, because a message has to cross the pipe, costing bandwidth and CPU, before its legitimacy can be determined. In any case, a domain that uses these mechanisms still can use SPF to announce that any messages without a signature should be rejected.

Challenge/response: you don't want to send challenges to spam, especially not forged spam. If SPF tells you a sender address definitely was forged, you can junk the message without bothering to challenge it.