The Null Device

The latest weapon in the war on spam is SPF; this is essentially a whitelisting technique which uses DNS servers to check whether a message really comes from the domain in the From: address. The way it works is essentially this: when a machine with IP address 123.45.67.89 connects to a mail relay to send a message, whose sender address says it's From: bob@schmoop.com, it looks up the DNS info on schmoop.com, and checks for a SPF entry. If there is one, it checks to see whether 123.45.67.89 is a legitimate schmoop.com host. If it isn't, it can reject the mail or bump up its SpamAssassin score or take some other action.

Of course, like all enhanced security measures, SPF removes convenience; for one, using a .forward file to forward mail from one account to another will fall foul of SPF, as the mail's origin (your other account) and its From: address (the sender) typically won't match (unless all your friends are on your local machine anyway). Secondly, putting an email address other than the one your ISP gives you (or one in a domain belonging to whoever runs your SMTP server) could result in your mail being rejected; which is a problem if you have a mail alias at a k3wl vanity domain but don't use that domain's SMTP servers for sending your mail.

¶ spam spf tech 9