Sender Permitted From.

SMTP SPF: Senders Permitted From is a spam-avoiding proposal that looks like it might be adopted widely; it gives senders of e-mail a way to describe through DNS records what their messages look like and where they come from. E-Mails that don’t match the description can then be discarded.The proposal is likely to be adopted widely because it creates interesting incentives: If significant e-mail receivers apply SPF checks where available (and don’t require senders to use SPF), and require existing sender domain names, this creates incentives for spammers to abuse non-SPF-enabled domain names. This will be painful for the holders and users of these domains, who in turn have strong incentives to publish SPF records. The same incentives apply for SPF records that are so loose that they are ineffective, or almost ineffective.Later: Steve Bellovin (on IP) has a number of problems with the scheme.