SPF, DKIM, and DMARC together — why the missing DMARC record was blocking registration emails

Background Registration confirmation emails were not reliably reaching users on Gmail and Outlook outside Japan — sometimes landing in spam, sometimes not arriving at all. Investigation pointed to a single root cause: the wpmm.jp domain had SPF and DKIM configured, but no DMARC record . What each of the three does SPF (Sender Policy Framework) declares in DNS which IP addresses are authorized to send mail for a domain. Receiving servers check the sending IP against the SPF record to confirm the source is legitimate. DKIM (DomainKeys Identified Mail) adds a cryptographic signature to the message headers and body. The receiving server looks up the public key in DNS and verifies that the message has not been tampered with and was signed by a party controlling that domain. DMARC (Domain-based Message Authentication, Reporting and Conformance) sits above both. It tells receiving servers what to do when SPF and DKIM alignment fails, and it collects aggregate reports about how mail from the domain is being treated. The key point is that SPF and DKIM are independent checks. Without DMARC, there is no single authoritative statement about how the alignment result should influence delivery decisions. Major providers including Gmail weigh the absence of DMARC when scoring incoming mail. Adding the DMARC record The following TXT record was added to the wpmm.jp DNS: _dmarc.wpmm.jp TXT "v=DMARC1; p=none; rua=mailto:info@wpmm.jp" p=none means "collect data, but do not reject or quarantine mail that fails alignment." Starting with p=reject or p=quarantine risks blocking legitimate mail if DKIM alignment turns out to be misconfigured somewhere. The safe approach is to start with p=none , monitor the reports, and tighten the policy gradually. rua=mailto:info@wpmm.jp sets the destination for aggregate reports. Google and other receivers periodically send XML summaries showing which mail passed or failed SPF/DKIM alignment. This moves visibility from passive (you notice when users compl