Sender ID Framework (SIDF) and DomainKeys Identified Mail (DKIM) are the two primary authentication proposals that vendors have grouped around for e-mail authentication. Some companies are already using one or both approaches in trials or complete implementations in their e-mail systems. Both technologies, however, need to be more powerful. In addition, industry experts say authentication alone will probably not extensively limit spam and phishing and will serve only as one tool to combat spam and phishing. Companies have been using such approaches as content filtering to identify spam, along with blacklists, retyping of keywords by senders, requirements for a human response, and use of reputation services through which an e-mail senders actions and history can indicate whether or not messages from that user might be spam or phishing. However, they do not deal with e-mail address spoofing and other problems related to Simple Mail Transfer Protocol's (SMTP's) inability to sufficiently authenticate the identity of a sender. SIDF blends Microsoft Caller ID technology with Sender Policy Framework (SPF, previously known as Sender Permitted Form). With SIDF, Internet Service Providers (ISPs) and companies register their mail server's IP addresses with the IANN and addresses are stored in DNS databases. DKIM is the most widely used cryptographic-based e-mail authentication approach and combines the Yahoo!-supported DomainKeys and the Cisco-supported Identified Internet Mail. An analyst for Forrester believes that SIDF is simpler and will be widely used before DKIM, which could need five years to move substantially into the market.