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To reduce phishing and account-takeover risk in Microsoft 365, protect sign-ins with multifactor authentication (MFA), block legacy authentication, authenticate every domain that sends mail for your organization, and verify that email threat policies cover the right recipients. These controls reduce risk; they cannot guarantee every phishing message will be blocked. Microsoft recommends investigating delivered phish and tuning policies when needed.
Choose an identity baseline: Security Defaults or Conditional Access
Start by checking what is already enabled in your tenant. Microsoft says new tenants receive Security Defaults by default, but that is not a reason to assume the setting is still on. Security Defaults provide a preconfigured baseline; Conditional Access offers more control for organizations with the required licensing. Microsoft cautions against turning off Security Defaults without replacing their protections. Microsoft’s Security Defaults guidance and its MFA setup guidance explain the options.
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| Option | Licensing | Customization | Operational considerations |
|---|---|---|---|
| Security Defaults | Available with the free Microsoft Entra tier. | Preconfigured baseline; no policy customization. | Simpler to operate. Includes MFA-related protections and blocks legacy authentication. |
| Conditional Access | Requires Microsoft Entra ID P1 or P2. Microsoft’s setup page gives examples including Microsoft 365 Business Premium and E3 with P1, and E5 with P2; verify current subscription entitlements. | Supports more granular, customized rules. | Requires policy design, testing, and ongoing review. Security Defaults and Conditional Access cannot be enabled simultaneously. |
If you move from Security Defaults to Conditional Access, first recreate the protections you rely on: require MFA for users, require MFA for administrators, block legacy authentication, and require MFA for Azure management. Do not disable the existing baseline until its replacement is ready.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsRequire MFA and block legacy authentication
Security Defaults prompt users to register for MFA, require MFA for administrators, require MFA for users when needed, and block legacy authentication. They also block device-code flow and protect privileged activities such as Azure management. The user-facing behavior is not necessarily an MFA prompt at every sign-in.
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With Conditional Access, create and test policies that require MFA for users and administrators and block legacy authentication. Older protocols and clients—including IMAP, SMTP, and POP3 in relevant configurations—may not support MFA. A policy that requires MFA does not make an unsupported legacy sign-in safe: blocking that authentication path is a separate protection. Before enforcement, identify devices and applications that still depend on legacy protocols, then replace or reconfigure them rather than leaving a broad exception.
Give administrator accounts phishing-resistant MFA
For privileged roles such as Global Administrator, Exchange Administrator, Security Administrator, and Conditional Access Administrator, Microsoft recommends phishing-resistant MFA. FIDO2 security keys are one possible method, but compatibility and setup depend on the tenant and the organization’s sign-in arrangements.
- Identify privileged accounts and confirm which phishing-resistant sign-in methods they can use.
- Register the compatible methods before enforcing the policy, and preserve emergency access accounts by excluding them as Microsoft advises.
- Use report-only mode to assess who would be affected, resolve access or compatibility issues, and then enforce the policy.
Follow Microsoft’s phishing-resistant MFA guidance for administrator roles when configuring the policy.
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Authenticate every domain that sends your organization’s mail
Configure SPF, DKIM, and DMARC in DNS for every domain you own that sends mail. Include third-party platforms that send on your organization’s behalf, not just Microsoft 365.
- SPF identifies the services authorized to send mail for a domain.
- DKIM adds a signature that receiving systems can check.
- DMARC lets the domain owner publish handling instructions for messages that fail authentication.
Incorrect or incomplete authentication can cause legitimate messages to go to Junk or quarantine. Avoid using broad allowlists to hide a domain-authentication or delivery problem. See Microsoft’s recommended settings for EOP and Defender for Office 365.
Check which email protections apply to each recipient
All Microsoft 365 organizations with cloud mailboxes receive baseline anti-phishing protection and spoof intelligence. Defender for Office 365 adds options including user and domain impersonation protection, configurable phishing thresholds, Safe Links, and Safe Attachments. Availability depends on licensing, and the default anti-phishing policy does not automatically configure every impersonation feature. Review policy assignments and confirm that the users and domains at greatest risk are covered.
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- WORKS WITH 1000+ ACCOUNTS: Compatible with popular accounts like Google, Microsoft, and Apple. A single YubiKey 5 NFC secures 100+ of your favorite accounts, including email, password managers, and more
- FAST & CONVENIENT LOGIN: Plug in your YubiKey 5 NFC via USB and tap it, or tap it against your phone (NFC), to authenticate. No batteries, no internet connection, and no extra fees required
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- PRIMARY & SPARE KEYS: Just like having a spare house key, we recommend buying two YubiKeys - one for daily use and one as a spare. That way you’ll never get locked out of your accounts
Microsoft documents these phishing-threshold values: level 1 for the default, level 3 for Standard, and level 4 for Strict. They describe Microsoft’s settings, not a guarantee that the most aggressive option is best for every organization. Review the effect on legitimate mail before applying a more aggressive policy broadly.
| Protection profile | Recipient scope | What to know |
|---|---|---|
| Built-in protection | Assigned to all recipients by default, subject to exceptions; supplies Safe Links and Safe Attachments coverage to recipients not assigned to Standard, Strict, or applicable custom policies. | Provides coverage for otherwise uncovered recipients; verify exceptions and assignments. |
| Standard preset | Only recipients assigned to the preset. | A baseline Microsoft describes as suitable for most users. It does not apply to anyone until enabled and assigned. |
| Strict preset | Only recipients assigned to the preset. | More aggressive; consider it for selected high-value or priority users after assessing impact. It does not apply until enabled and assigned. |
Preset security policies take precedence over default and custom threat policies. Check for overlap and ordering rather than assuming a custom policy will override a preset. Microsoft’s preset security policy documentation describes assignment and coverage.
Microsoft says malware and high-confidence phishing are quarantined by default under its secure-by-default protections, and some overrides do not apply to those detections. Broad safe-sender or allowed-domain exceptions can weaken protection. If a non-Microsoft service sits in front of Microsoft 365, review Microsoft’s secure-by-default guidance and the routing caveats before relying on this behavior.
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- WORKS WITH 1000+ ACCOUNTS: Compatible with Google, Microsoft, and Apple. A single Security Key NFC secures 100 of your favorite accounts, including email, password managers, and more.
- FAST & CONVENIENT LOGIN: Plug in your Security Key NFC via USB-A and tap it, or tap it against your phone (NFC) to authenticate. No batteries, no internet connection, and no extra fees required.
- TRUSTED PASSKEY TECHNOLOGY: Uses the latest passkey standards (FIDO2/WebAuthn & FIDO U2F) but does not support One-Time Passwords. For complex needs, check out the YubiKey 5 Series.
- BUILT TO LAST: Made from tough, waterproof, and crush-resistant materials. Manufactured in Sweden and programmed in the USA with the highest security standards.
Investigate a phishing message that gets through
A delivered message is a reason to inspect what happened, not to create a broad allowlist. Microsoft recommends examining message headers, reporting suspicious mail through the Submissions page, reviewing spoof and impersonation insights where licensed, and checking for compromised accounts and malicious inbox forwarding rules.
- Inspect the
X-Forefront-Antispam-Reportheader and its Spam Filtering Verdict (SFV) value. Microsoft’s exampleSFV:SKNindicates that a mail-flow rule skipped spam filtering. - Report the message through the Submissions page and review available spoof and impersonation insights.
- Check whether the same message reached other recipients, then investigate any account that may have been compromised, including its inbox forwarding rules.
- Correct the policy, authentication, or mail-flow configuration that allowed the message through. Do not add your own domains to broad allowed-sender lists as a quick fix.
Microsoft’s anti-phishing tuning guidance covers investigation and policy tuning. For additional context on how Microsoft 365 handles spoofed mail, see its anti-spoofing protection documentation.
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