DomainKeys Identified Mail (DKIM) is a system that helps authenticate your emails. Much like SPF, DKIM is an authentication framework that tells the world that it is really you sending a given email message. While SPF uses a DNS TXT record to list authorized sending IP addresses for a given domain then, DKIM lets you add a digital signature to email messages in the message header. DKIM is effectively a check of “I am who I say I am” as an email sender.
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Simply, DKIM verifies that Act-On is allowed to send emails on your behalf. If Act-On is not set up with the appropriate DKIM keys, most email servers assume that we do not have permission to send your emails. DKIM authentication does not guarantee inbox deliverability, but without it, emails are very likely to be sent to spam or rejected altogether.
To set up DKIM, please see Email From Setup.