M365: Essential Day One Checklist

Key insights

• Day-One Audit: Run a full tenant audit immediately after onboarding to find gaps before changes hide risks.
  Check identity, authentication, policy deployment, and recent alerts so you can prioritise fixes fast.
• Microsoft 365 Admin Takeover: Use the official takeover flow for an unmanaged tenant to regain admin rights by proving domain ownership.
  This process creates a user context, guides you to the admin experience, and grants admin access after verification.
• DNS TXT Verification: Add a DNS TXT record to prove you control the custom domain — this is the main verification step in the takeover flow.
  You usually start by creating an account in the tenant via a self-service workload, then follow the takeover wizard to validate the TXT record.
• Conditional Access: Review and document existing Conditional Access policies on day one to confirm coverage for risky sign-ins and legacy authentication.
  Ensure policies are visible, correctly scoped, and deployed where needed, then test important sign-in paths.
• MFA & Security Baseline: Verify Multi-Factor Authentication status and run a security-baseline checklist to spot weak controls.
  Enable or enforce MFA where missing, apply baseline hardening, and configure alerting for critical events.
• Managed vs Unmanaged Tenant: The takeover wizard only applies to truly unmanaged tenants; if a partner or CSP already manages the tenant, open support or plan a migration instead.
  Gather business proof (company records, billing, domain control) ahead of time to speed support and escalation when needed.

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