Domain Keys Identified Mail (DKIM) is the technology that helps to reduce spam and phishing. It allows companies to vouch for their email messages. Technically, it can be done with the help of a cryptographic signature linked to a domain name instead of the traditional IP address.
For example, if you deal with banking and you sent mailings from the address email@example.com, to prevent your subscribers from spamming and phishing you should prove that domain, "vipbank.com," belongs to you. If you don’t set DKIM there is evidence that email services like Gmail, Yahoo, and Hotmail deliver your mailings to the spam folder. It happens because there are a lot of attempts to steal users’ passwords or financial details from email messages. Phishers often send emails on behalf of large companies and ask users to "verify their account" or "confirm their billing information," and then use the subscribers’ accounts for fraudulent purposes.
DKIM technology determines who the sender of an email is. That means that the email you send will not be regarded as an email from phishers, and so email services will deliver it to the "inbox." The sender authentication with DKIM is implemented by a digital (cryptographic) signature – a hidden code that is added to the email. The signature confirms that the email was sent from an email address in the particular domain.
Read more about DKIM in this article.