Microsoft 365 Security Best Practices for Small Businesses

A practical, prioritized checklist for securing a small business Microsoft 365 tenant: identity, email, devices, data, and monitoring.

Checklist · By NexJen IT Solutions · Reviewed October 2026 · 4 min read
Key takeaways
Start with identity: enforce MFA for everyone and block legacy sign-ins.
Then harden email, protect devices, control sharing, and watch the logs, in roughly that order of impact.
If you only do three things: MFA everywhere, endpoint detection and response on every computer, and an independent Microsoft 365 backup.

Microsoft 365 includes strong security features, but many of them are off, half-configured or left at defaults that suit nobody in particular. This checklist is ordered by impact, so you can start at the top and work down.

1. Lock down identity first

• Enforce multi-factor authentication for every user, using Security Defaults or, on Business Premium, Conditional Access.
• Block legacy authentication protocols that cannot do MFA.
• Use separate admin accounts for administrative work, and keep the number of Global Administrators small.
• Require stronger, phishing-resistant MFA methods for admins where possible.
• Review guest and external accounts regularly and remove the ones you no longer need.

2. Harden email

• Publish SPF, DKIM and DMARC records so others can verify your mail and spoofing of your domain is harder.
• Turn on the anti-phishing and safe-link protections available in your plan; Business Premium includes Defender for Office 365 Plan 1.
• Disable automatic forwarding of email to outside addresses unless there is a business need.
• Train staff to verify payment and bank-detail changes by phone, using a number you already have.

3. Protect every device

• Run endpoint protection with detection and response on every computer, not just antivirus.
• Keep Windows, macOS and applications patched automatically.
• Encrypt laptops so a lost device does not mean lost data.
• Decide how personal phones and computers may access company email and files, and enforce it.

4. Control sharing and data

• Set sensible defaults for SharePoint and OneDrive external sharing, such as specific people rather than anyone with the link.
• Organize files into SharePoint sites with clear owners instead of everything living in personal OneDrives.
• Use sensitivity labels or data loss prevention for client data if your plan includes them.
• Back up Exchange, OneDrive, SharePoint and Teams with a separate service, and test restores.

5. Watch and review

• Check Microsoft Secure Score periodically; it lists improvement actions in priority order.
• Make sure audit logging is turned on, so you can investigate if something goes wrong.
• Review sign-in risk and alerts, or have someone responsible for doing so.
• Have a written plan for what to do in a security incident, including who to call.

6. Offboard people properly

When someone leaves, block sign-in, revoke sessions, remove their MFA methods and devices, transfer their files and mailbox to the right person, and remove licenses only after their data is safe. A checklist makes this consistent.

Where to start if this feels like a lot

If you do only three things this month, enforce MFA everywhere, put endpoint protection with detection and response on every computer, and confirm your Microsoft 365 data is backed up by a separate service. Those three close most of the doors attackers use against small businesses.

How we can help

NexJen Essentials applies a security baseline matched to your Microsoft 365 plan, with MFA, endpoint protection with managed detection and response, and Microsoft 365 backup included. If you would rather keep your current IT arrangement, our fixed-fee Security Posture and Compliance Assessment benchmarks your tenant against the CIS Benchmark and Microsoft Secure Score and gives you a written, prioritized report. It maps your gaps; it is not a certification or audit.

Common questions

What is the most important Microsoft 365 security setting?

Enforcing multi-factor authentication for every user. Microsoft's research shows MFA reduces the risk of account compromise by more than 99.2%.

What is Microsoft Secure Score?

A measurement in the Microsoft Defender portal of how well your tenant uses available security features, with a prioritized list of improvement actions.

Do I need Business Premium to secure Microsoft 365?

No, but it helps. Every plan can enforce MFA with Security Defaults. Business Premium adds Conditional Access, Intune device management and Defender for Business.

How often should Microsoft 365 security be reviewed?

Settings and threats change, so review regularly. A managed provider should monitor continuously; otherwise schedule at least an annual review.

Sources
Security defaults in Microsoft Entra ID (Microsoft Learn)How effective is multifactor authentication at deterring cyberattacks? (Microsoft Research)Email authentication in cloud organizations (Microsoft Learn)Microsoft Secure Score (Microsoft Learn)CIS Microsoft 365 Benchmarks (Center for Internet Security)

Want help prioritizing?

Start with a free Security Snapshot, or ask about our Security Posture & Compliance Assessment for a written, risk-rated review.

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