Danger: Authenticating e-mail can break it (ZDNet)
There are two main ways of authenticating e-mail: Sender ID and DomainKeys Identified Mail, or DKIM. Backed by Yahoo and Cisco Systems, DKIM relies on public key cryptography. It attaches a digital signature to outgoing e-mail, so recipients can verify that the message comes from its claimed source. Sender ID is further along in adoption than DKIM. It requires Internet service providers, companies and other Internet domain holders to publish SPF (Sender Policy Framework) records to identify their mail servers. This usually does not require new hardware or software; the most arduous part is doing an inventory of mail servers and the subsequent maintenance of that record."