The primary purpose of email authentication is to verify that the sender of the message is in fact the party responsible for the message being sent. A few types of email authentication also help to protect against issues like domain spoofing and phishing attacks.
DKIM is a method of email authentication that has been around for a few years and has become increasingly popular with many ISPs looking for a cryptographic solution to the email forgery issue. Recent evidence indicates that many ISPs are now using it to filter mail based on whether the sender authenticates or not – meaning in this case that a sender adds a digital signature within the email header that is verified against a published key for the domain or user in question.
CakeMail has been using DKIM as one of our methods of authenticating emails since our inception. After several recent tests using the same content and sending emails from the same IP, the CakeMail Delivery Team saw a distinct increase in email being sent to the inbox (with images and links enabled) that would have otherwise gone to junk. Although most, if not all, of the large ISPs are verifying using DKIM, we saw the best results at Gmail and Yahoo!.