DKIM complements SPF, an anti-spam email validation framework, by providing email senders with a way to digitally sign all outgoing email from their domain.
DKIM-signed messages can pass through email forwarding servers with their signatures intact. Like SPF, it requires no changes in behavior for end users. This makes DKIM and SPF much easier to deploy than S/MIME, and as a result they have been widely adopted by the world’s major email box providers.
DKIM allows recipients to confirm sender identity and determine whether the message was altered during transit. But with just SPF and DKIM, there is no way for a recipient system to know how much reliance they should place on email validation.
There is also no policy framework—meaning the instructions are not there to indicate what to do with messages that fail authentication, and no feedback mechanism to let senders know what actions were performed.
Together, SPF and DKIM provide an important framework to fight spam and ensure the integrity of the email. DMARC acts as an overlay on this framework and adds three key elements:
DMARC is the only way for email senders to tell email recipients that emails they are sending are truly from them.
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