[Complete Guide] Japan’s Ministry of Internal Affairs Pushes DMARC Adoption — Understand “What, Why, and How” of Email Authentication in the Age of Generative AI Phishing (2025 Edition)

Key Points First (Inverted Pyramid)

• Domestic Update: As generative AI makes Japanese phishing emails more convincing, Japan’s Ministry of Internal Affairs (MIC) has urged industries to adopt DMARC and other email authentication technologies. DMARC deployment is now a government-endorsed anti-fraud measure across agencies.
• What is DMARC?: A protocol that aligns SPF and DKIM results with the domain in the “From” header, and declares how to handle misaligned emails (none/quarantine/reject) via DNS. It also provides aggregate (rua) and forensic (ruf) reporting for tuning.
• Related Technologies: SPF/DKIM (prerequisites), ARC (for forwarded messages), MTA-STS/TLS-RPT (TLS enforcement for delivery), and BIMI (brand logo display, requires DMARC with p=quarantine/reject).
• Why is it needed?: Generative AI has made phishing emails in fluent Japanese common. Domain spoofing is the decisive trick. Raising DMARC to p=reject is seen as the only effective countermeasure in Japanese reports.
• Why are you a target?: Email remains the most important business touchpoint with high ROI. Finance, e-commerce, HR, logistics, and supply chains are especially targeted. The nuances of polite Japanese make fake messages more convincing, and attackers get results just by impersonating a brand.

1｜Background: MIC’s Strong Push for Adoption

On September 1, 2025, Japan’s MIC officially called for stronger implementation of DMARC as a countermeasure against sophisticated phishing emails powered by generative AI. Since 2024, the government has positioned DMARC as a core part of its National Anti-Fraud Strategy, alongside initiatives from the Anti-Spam Council and the Anti-Phishing Council. It’s being advanced in parallel with smishing (SMS-based phishing) countermeasures.

Core Message: It’s not just about “AI writing better emails.” The real issue is that the success rate and frequency of phishing attacks have increased. Against spoofing that pretends to be from a legitimate domain, DMARC lets you isolate or reject those emails, and is thus the fastest and most practical way to reduce real damage.

2｜What is DMARC? — A System for Declaring Domain Alignment and Handling Policy

2-1. In One Sentence

DMARC (Domain-based Message Authentication, Reporting & Conformance) lets domain owners align SPF/DKIM results with the From header domain, and declare the handling policy (none/quarantine/reject) via DNS. It also visualizes your entire sending landscape through aggregate and forensic reports.

2-2. Basic Relationship Diagram

• SPF: Publishes which IP addresses are authorized to send for a domain.
• DKIM: Adds a cryptographic signature to the message, verified with a public key.
• DMARC: Checks if the From header domain aligns with SPF or DKIM, and applies the specified policy.

Important: If either SPF or DKIM passes and is aligned with the From domain, DMARC passes. Because SPF can break during forwarding, many Japanese guidelines recommend DKIM prioritization.

2-3. Sample DMARC Record (DNS TXT)

v=DMARC1; p=none; rua=mailto:dmarc-agg@your.example; ruf=mailto:dmarc-afr@your.example;
fo=1; pct=100; aspf=s; adkim=s; sp=quarantine

• p=: none → quarantine → reject (gradual strengthening)
• rua/ruf: Recipients for aggregate and forensic reports
• aspf/adkim: Alignment strictness (s = strict, r = relaxed)
• pct: Percentage of mail affected (useful for phased rollout)
• sp: Policy for subdomains (commonly forgotten)

Japanese guidelines recommend: Start with p=none for monitoring → fix misalignments → move to quarantine → finally to reject. Staying at none is not effective; reject is the goal.

3｜Related Technologies — Building a Unified Defense

1. SPF/DKIM (Foundational): Authorizes sender IPs and adds signatures. Needs alignment even for third-party senders (e.g., MA tools, CRM, invoicing SaaS). Inventorying and configuring each vendor is critical.
2. ARC (Authenticated Received Chain): Helps preserve trust through forwards or mailing lists that might break SPF. Complements DMARC.
3. MTA-STS/TLS-RPT: Enforces TLS for email transport and provides reporting to counter eavesdropping and downgrade attacks.
4. BIMI (Brand Indicators for Message Identification): Requires DMARC (p=quarantine or reject). Allows brand logos to be shown in inboxes (e.g., Gmail). Requires VMC/CMC certificates. Enhances security, deliverability, and trust.

4｜Why Is DMARC Necessary? — Four Realities of the AI Era

4-1. AI Has Made Emails Convincing

AI-generated emails in Japanese are fluent and tailored, eliminating obvious red flags like typos. We now need mechanical domain authenticity checks.

4-2. Spoofing Is Too Cost-Effective

Email is the cheapest attack channel. Fraud using fake invoices, redelivery notices, or bank detail changes is rampant. Industries like finance, e-commerce, HR, logistics, and SaaS alerts are prime targets. Rejecting fake senders is the first defense line.

4-3. Many Only Implement “Monitoring”

While adoption is increasing, many organizations stop at p=none, which only observes but doesn’t protect. Raising to quarantine and reject is now a national recommendation.

4-4. Adoption Has Clear Guidance Now

With government support, police and municipality campaigns, and industry guidelines, there’s now standardized implementation guidance, including how to read rua/ruf reports.

5｜Why Are You a Target? — From the Attacker’s Perspective

1. Brand Trust Becomes a Weapon: Posing as notifications like account registration, billing, redelivery, or HR updates drives high open rates.
2. Japanese Formal Language: AI mimics polite tones and internal company context, increasing click-through on attachments or links.
3. Supply Chain Trust Transfers: Emails from vendors to accounting, or contractors to IT often pass without suspicion. DMARC alignment must cover partners too.
4. Blended SMS/Email Attacks: Combine SMS links with invoice emails. DMARC-based email rejection becomes the first shield.

6｜Practical: 30-Day Roadmap Toward p=reject

Week 1｜Inventory (Map Out Senders)

• List From domains and subdomains (e.g., example.co.jp, mail., billing., recruit.)
• Identify all sending paths: internal MTA, cloud MA, billing SaaS, CRM, ATS, etc.
• Audit current SPF/DKIM settings and key lengths. Prioritize DKIM.

Week 2｜Monitoring (p=none + Reports)

• Publish DMARC with p=none; rua=…; ruf=…; sp=quarantine
• Use analysis tools to read rua (aggregate) and ruf (forensic) reports. Identify misalignments by third parties.

Week 3｜Correction (Achieve Alignment)

• Align SPF authorizations, DKIM keys, and signing domains (d=) for third-party senders.
• If mail forwarding is common, consider ARC support.
• Prepare for BIMI: requires p=quarantine+ and verified logo certificates (VMC/CMC).

Week 4｜Enforcement (p=quarantine → p=reject)

• Start with pct=50 to monitor possible false positives.
• If stable, move to p=reject and sp=reject for subdomain control.
• Update operational manuals for periodic key rotation and vendor onboarding.

Reminder: “DMARC implemented” is not equal to “protected”. Only reaching reject provides real protection.

7｜DNS Record Examples (Minimum Viable Set)

7-1. SPF (TXT)

example.co.jp. IN TXT "v=spf1 include:_spf.mailvendor.example include:_spf.crm.example ip4:203.0.113.10 ~all"

• Use include for third-party senders. Start with ~all, then move to -all after validation.

7-2. DKIM (Public Key)

selector1._domainkey.example.co.jp. IN TXT "v=DKIM1; k=rsa; p=MIIBI...IDAQAB"

• Use 2048-bit or higher keys. Multiple selectors allow easy key rotation.

7-3. DMARC (TXT)

_dmarc.example.co.jp. IN TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc-agg@security.example; ruf=mailto:dmarc-afr@security.example; fo=1; aspf=s; adkim=s; sp=quarantine; pct=50"

• Use strict alignment (aspf=s; adkim=s) to tighten spoofing tolerance.
• After validation, switch to pct=100 and p=reject.

8｜Optional but Powerful: BIMI to Visualize Trust

• Requirements: SPF, DKIM, and DMARC with p=quarantine/reject in place. Also need SVG logo and VMC/CMC (depends on recipient email provider).
• Benefits: Displays verified brand logos in inboxes, boosting trust and open rates.

9｜Common Pitfalls (and How to Avoid Them)

1. Leaving p=none indefinitely: Detects spoofing but doesn’t stop it. Use pct to gradually enforce.
2. Forgetting subdomain policy: Without sp=, gaps remain, especially in customer-facing domains.
3. SPF breaks on forwarding: Use ARC or DKIM prioritization. Account for mailing list re-writes.
4. Unaligned third-party vendors: Use rua/ruf to detect issues. Include domain alignment clauses in contracts.
5. Misunderstanding BIMI: Requires prior DMARC implementation. Also verify VMC/CMC and recipient policies.

10｜Tailored Benefits & KPIs (by Department)

• Executives / PR: Prevent brand impersonation lawsuits or reputational damage. Use BIMI to show authenticity. KPIs: Reduced spoof incidents / deliverability / inquiry types.
• IT / SOC: Use rua/ruf to discover unknown senders. Operationalize vendor audits and key rotations. KPIs: DMARC pass rate / reject counts / false positive rate.
• Marketing / CRM: Align vendor configs to improve engagement and inbox placement. BIMI enhances visibility. KPIs: Inbox placement / open rate / complaint rate.
• Legal / Compliance: Align with government guidance, integrate into internal policies, and improve RFP scores. KPIs: Zero audit findings / published policy & test results.

11｜Sample Internal Operation Flow (with Notification Template)

1. Policy: “Our company implements SPF/DKIM/DMARC, and uses p=reject to block spoofed emails.”
2. Steps:
   • Inventory senders → Configure SPF/DKIM → Publish DMARC (p=none) → Monitor reports → Move to quarantine → Move to reject
3. Vendor Contract Clauses:
   • “Ensure DKIM signature domain aligns with From domain”
   • “Use 2048-bit+ keys”
   • “Share monthly rua summaries”
4. Incident Response:
   • If mail is “not delivered”, check From vs d= alignment, ARC presence, and recipient-side DMARC evaluation logs.

12｜Conclusion: DMARC Is a Must-Have in the Age of AI

• In a world where convincing fake Japanese emails are common, DMARC is both the first step and the final foundation for proving sender authenticity.
• Staying at p=none is not a defense. Move step-by-step to quarantine and reject.
• Align third-party senders, implement ARC for forwarding, and visualize legitimacy via BIMI. Government support means the time to act is now.

References (Authoritative Sources)

• News: MIC calls for stronger DMARC adoption due to AI-enhanced phishing.
• Government Policy: DMARC adoption promoted as part of national anti-fraud strategy.
• Reports: Japanese studies highlight p=reject as the only effective protection.
• Guidelines: DMARC adoption guide (Anti-Spam Council of Japan).
• Technical Basis: DMARC builds on SPF/DKIM with alignment and policy declaration (dmarc.org).
• BIMI Requirements: Requires DMARC with p=quarantine/reject and verified certificates.
• Educational Materials: Explainers on DMARC and reject/quarantine from Japanese municipalities and police.