Sender Policy Framework (SPF) Overview
Similarly to DKIM, SPF verifies that Act-On is allowed to send emails on your behalf. If Act-On is not specified in the SPF entry, most email servers assume that we do not have permission to send your emails. SPF authentication does not guarantee inbox deliverability, but without it emails are very likely to by sent to spam or rejected altogether.
To set up SPF, please see Required Technical Setup.