Many companies invest time in correctly setting up SPF and DKIM – only to find that emails still land in the recipient's spam folder or aren't delivered at all. This causes uncertainty, as these mechanisms are considered central pillars of email security.
The explanation lies in the fact that while SPF and DKIM are necessary, they are not sufficient. Email deliverability is not a single switch, but the result of several factors evaluated together by receiving mail servers.
This article classifies why authentication alone is not enough and which causes are particularly responsible for delivery problems in practice.
SPF and DKIM: Foundation, but not a Free Pass
SPF defines which servers are allowed to send emails on behalf of a domain. DKIM ensures that content has not been tampered with. Both help mail servers build trust in a message.
What they do not do: they do not guarantee delivery to the inbox. Providers like Google or Microsoft always evaluate emails in context – in conjunction with other technical and content signals.
Reputation: The Often Underestimated Factor
One of the most important variables is the reputation of the sending domain and IP address. Typical problems arise when emails are suddenly sent in large volumes or if many recipients mark messages as spam.
Even if SPF and DKIM are correct, a poor reputation will still lead to poor deliverability.
Content Signals and Technical Consistency
Exaggerated subject lines, too many links, or large images can make spam filters suspicious. Furthermore, if the sender address, domain, and return path do not align cleanly, an inconsistent picture emerges despite formal authentication.
DMARC: The Missing Link
In many cases, DMARC is missing or only rudimentarily configured. DMARC defines how servers should handle emails when SPF or DKIM do not clearly pass. Without it, the likelihood of landing in spam increases. Learn more here: Email Security & Deliverability.
Integrated Infrastructure
Delivery problems often arise at the interfaces where domain, DNS, and mail systems do not mesh properly. A structured environment simplifies analysis: Domains in Switzerland.
Conclusion
SPF and DKIM are indispensable but no guarantee for perfect delivery. Only the interplay with DMARC, clean sending behavior, and consistent technical configuration builds lasting trust.
