Email Security Quick Wins: DMARC, Phishing Controls & Hardening

Why Email Security Matters More Than Ever

Despite years of security awareness training, email remains the primary attack vector for ransomware, business email compromise (BEC), and credential theft. The reason is simple: attackers follow the path of least resistance, and email lands directly in front of your employees.

The good news? A handful of configuration changes can dramatically reduce your exposure. This isn’t about buying new tools—it’s about properly configuring the ones you already have.

Part 1: Email Authentication (DMARC, SPF, DKIM)

Email authentication prevents attackers from spoofing your domain to trick customers, partners, or employees. If you haven’t implemented DMARC, SPF, and DKIM, you’re leaving the door open for impersonation attacks.

SPF (Sender Policy Framework)

SPF specifies which mail servers are authorized to send email on behalf of your domain.

• Create a TXT record in your DNS
• List all legitimate sending sources (your mail server, marketing platforms, CRM)
• End with -all (hard fail) or ~all (soft fail) for unauthorized senders

DKIM (DomainKeys Identified Mail)

DKIM adds a digital signature to outgoing emails, proving they haven’t been tampered with in transit.

• Generate DKIM keys in your email platform (Microsoft 365 or Google Workspace)
• Publish the public key as a DNS TXT record
• Enable DKIM signing for outbound mail

DMARC (Domain-based Message Authentication)

DMARC ties SPF and DKIM together and tells receiving servers what to do when authentication fails.

• Start with p=none to monitor without affecting delivery
• Review DMARC reports to identify legitimate senders you missed
• Progress to p=quarantine then p=reject as you gain confidence

“DMARC at ‘reject’ policy stops attackers from impersonating your domain. It’s one of the highest-impact security controls you can implement.”

Part 2: Phishing and BEC Controls

Email authentication prevents spoofing of your domain, but you also need controls for inbound threats—phishing emails that target your employees.

Safe Links and Safe Attachments

Both Microsoft 365 and Google Workspace offer URL scanning and attachment sandboxing:

• Microsoft 365: Enable Safe Links and Safe Attachments in Defender for Office 365
• Google Workspace: Enable Security sandbox for attachments and Enhanced pre-delivery message scanning

External Sender Warnings

Add visual warnings to emails from outside your organization:

• External email tags (“[EXTERNAL]” in subject or banner)
• First-time sender warnings
• Domain impersonation alerts (lookalike domains)

Impersonation Protection

Configure anti-impersonation rules for high-value targets:

• Protect executive names from spoofing
• Flag emails from domains that look similar to yours
• Block emails that impersonate your own domain internally

Part 3: Secure Collaboration Defaults

Email and collaboration tools are intertwined. Lock down sharing defaults to prevent data leakage:

Microsoft 365

• Default sharing links to “People in your organization”
• Require authentication for anonymous links
• Set expiration dates on external sharing links
• Block external forwarding of email (or require approval)

Google Workspace

• Default to “Restricted” sharing for new files
• Disable link sharing outside your organization (or require approval)
• Enable external sharing warnings
• Configure email allowlist/blocklist for external communication

Part 4: Evidence for Auditors

Security controls are only as good as your ability to prove they’re in place. Maintain evidence of:

• DMARC, SPF, and DKIM records (screenshots of DNS)
• Email security policy configurations (export from admin console)
• DMARC aggregate reports (showing enforcement)
• Phishing simulation results and training completion

Key Takeaways

• Implement DMARC, SPF, and DKIM to prevent domain spoofing
• Enable Safe Links/Attachments and sandbox scanning
• Add external sender warnings and impersonation protection
• Lock down collaboration sharing defaults
• Document configurations for audit readiness

Conclusion

Email security isn’t about buying more tools—it’s about configuring what you have correctly. Start with DMARC at monitoring mode, enable built-in phishing controls, and lock down sharing defaults. These quick wins dramatically reduce your attack surface without major investment.