DomainKeys Identified Mail (DKIM), an IETF standards-track signature-based mechanism for authenticating email messages, has seen significant growth in usage this year. DKIM is the result of combining Yahoo’s DomainKeys technology with Identified Internet Mail which was developed at Cisco. The following graphs tell much of the story:This counts the number of messages from domains other than cisco.com that are not classified as spam and have valid DKIM signatures. During this time, the amount of non-spam email received by Cisco has remained relatively constant. Some of this growth comes from large domains such as Gmail and Yahoo!. But much of this comes from many small domains that are also signing their messages. That story is told by the other graph:This graph counts the number of domains in a given week we have received at least one DKIM-signed message from. This has grown steadily as additional domains deploy DKIM signing. The big explosion of domains in April 2009 is due to a large email sending provider that uses a separate domain for each of their customers that has recently deployed DKIM signing.The important part of the story is only indirectly told by these graphs, however. It is the use of DKIM signatures to improve trust in email, and not the presence of the signatures themselves, that really matters. This is indirectly shown because the justification to deploy DKIM signing for many domains is that it may improve the domain’s deliverability (decreases the likelihood that the domain’s mail will be classified as spam by the recipient). Google and Yahoo! have announced that messages with valid DKIM signatures, where the domain has established a good reputation with them, are less likely to be classified as spam. This has real value to many senders, and it’s the real value proposition for DKIM: It provides a basis for more accurate evaluation of incoming mail by recipients.I’m proud of the industry’s progress in improving the trust landscape for email, and of Cisco’s participation in that effort.