Sender Policy Framework (SPF) Developed by Meng Wong, CTO and co-founder, Pobox, it is the simplest of the authentication solutions currently being tested and deployed in the marketplace, and involves publishing the sender’s IP or server address.
Sender ID Framework (SIDF) SIDF is an authentication solution that is the result of combining SPF with Microsoft Corp.’s proprietary authentication solution called Caller ID for Email. Microsoft will make SIDF available in Outlook and Exchange by the end of the year, it said. Caller ID authenticates the visible address in the "From" portion of an e-mail header. The complete solution, SIDF, adds an extra layer of protection against visibly forged domain names in phishing scams, in addition to the protection provided by SPF.
DomainKeys (DK) and Identified Internet Mail (IIM) DK and IIM are more advanced, cryptographic/signature-based solutions developed by Yahoo! and Cisco, respectively.
In June, Yahoo! and Cisco said they will merge the IIM specification with DK in order to create a stronger and more scalable cryptographic authentication solution called DomainKeys Identified Mail (DKIM).
Source: "Authentication, Accreditation & Reputation—For Marketers!" published by the Direct Marketing Association and Bigfoot Interactive, June 2005