5 Microsoft 365 Security Settings Every Business Should Check

Microsoft 365 is central to how many businesses communicate, collaborate and store information. That makes the security of the Microsoft 365 environment an important part of the wider business security picture.

1. Multifactor authentication (MFA)

Passwords can be stolen, reused or exposed through phishing. MFA adds another verification step, making a compromised password much less useful on its own.

Start with administrator accounts and then make sure ordinary user accounts are protected appropriately for the services they use.

Practical check: Review which accounts have MFA enabled, identify exceptions and understand why any exceptions exist.

2. Administrator accounts and privileged access

Administrative accounts can make significant changes to users, services, security settings and data. They should therefore be treated differently from ordinary day-to-day accounts.

A common improvement is to reduce unnecessary privilege and make sure administrative access is controlled, reviewed and used only when needed.

Practical check: Review your administrator list and ask whether every account still needs the permissions it currently has.

3. Legacy authentication and outdated access methods

Older authentication methods can create unnecessary security exposure because they may not support modern controls in the same way as current authentication approaches.

Businesses should understand which older protocols or applications still depend on legacy authentication before making changes, because disabling them without planning can disrupt legitimate users or systems.

Practical check: Identify applications, devices or workflows that still use older sign-in methods and plan a controlled move to modern authentication where possible.

4. Email security and anti-phishing controls

Email remains one of the most common starting points for impersonation, credential theft and malicious attachments. Microsoft 365 provides security features that can help, but they still need to be configured and reviewed.

Your wider email strategy should also consider domain authentication, message protection, suspicious-link handling and user awareness.

Practical check: Review your current email security configuration and make sure staff know how to report suspicious messages.

5. Device access and endpoint security

A secure account can still be at risk if a device is unmanaged, unpatched or poorly protected. This becomes especially important when employees work remotely or use multiple devices.

Review which devices can access company resources and whether they are appropriately protected, updated and managed.

Practical check: Know which business devices are accessing Microsoft 365 and have a clear process for new, lost, replaced and retired devices.

What else should you review?

The five areas above are a useful starting point, but Microsoft 365 security is broader than any single setting. Depending on your environment, you may also need to review:

• Conditional access and sign-in controls.
• External sharing and guest access.
• Mailbox forwarding and suspicious inbox rules.
• Security and audit logging.
• Retention and data protection requirements.
• Offboarding and access removal when staff leave.

A simple Microsoft 365 security checklist

1. Protect important accounts with MFA.
2. Review administrator roles and privileged access.
3. Identify outdated authentication dependencies.
4. Review email security and phishing protections.
5. Check which devices can access business resources.
6. Review access when employees join, change roles or leave.
7. Keep your configuration aligned with how the business actually works.
8. Review Conditional Access policies and test their exceptions.
9. Review guest users and external sharing.
10. Check suspicious mailbox forwarding and inbox rules.
11. Confirm audit and sign-in activity is reviewed by someone responsible.
12. Check that leavers no longer retain access.
13. Confirm the controls in use match the organisation's Microsoft 365 licensing.

Conditional Access: use it to reduce avoidable exposure

Conditional Access can be used to apply access rules based on circumstances such as the user, device, application, location and authentication strength. For businesses with the right Microsoft Entra licensing, this provides a more targeted approach than relying on a single organisation-wide setting.

Keep the design simple. Start with a small number of clearly defined policies for high-risk scenarios such as administrator access, unfamiliar sign-ins and access from devices that do not meet your organisation's requirements. Test policies with a small group before broad deployment so that legitimate work is not interrupted.

External sharing and guest access

Microsoft 365 collaboration can involve people outside your organisation, but external access should be intentional. Review guest accounts, shared links, Teams membership and SharePoint permissions regularly rather than assuming access disappears when a project ends.

A useful review asks three questions: who is the external person, why do they need access, and when should that access expire? Where a project has a defined end date, build an access-review step into the project close-down process.

Mailbox forwarding and suspicious inbox rules

Unexpected mailbox forwarding or unusual inbox rules can be an important warning sign after an account compromise. Attackers may attempt to hide messages, redirect copies to another address or create rules that interfere with normal communication.

Administrators should understand what forwarding is legitimately used for and investigate unexpected changes. For high-risk environments, include mailbox-rule and forwarding checks in routine security reviews.

Audit logs and security review

Logging is most useful when somebody knows which events matter and how they will be reviewed. Microsoft 365 and Microsoft Entra provide audit and sign-in information that can help identify unusual activity, but simply having logs available does not guarantee that an incident will be spotted.

Define a small set of events that should trigger attention, such as unexpected administrator changes, suspicious sign-ins, MFA changes, new application permissions or unusual mailbox activity. Keep the review process proportionate to the size and risk of the organisation.

Build security into joiner, mover and leaver processes

Microsoft 365 security is affected by staff movement. A person who changes role may need different access, while someone who leaves should no longer retain access to mailboxes, files, groups, applications or administration roles.

Document who is responsible for disabling accounts, transferring business information, removing sessions and reviewing delegated access. The exact workflow will vary between organisations, but the principle is consistent: access should follow the person's current business need rather than remain indefinitely.

Security features depend on licensing

Microsoft 365 security controls are not identical across all subscriptions. Some organisations have baseline protections, while others may have access to more advanced capabilities through Microsoft Entra ID, Microsoft Defender or additional Microsoft 365 licensing.

Before recommending a control, check what the organisation actually has licensed. This avoids designing a security process around features the business cannot use and helps identify where an upgrade would genuinely address a known risk.

A practical quarterly Microsoft 365 security review

A quarterly review is a useful starting point for many smaller businesses. The exact frequency should reflect the organisation's risk, regulatory obligations and pace of change.

1. Review Global Administrator and other privileged role assignments.
2. Check MFA coverage and investigate exceptions.
3. Review recent sign-in and authentication issues.
4. Check guest users, external sharing and project-specific access.
5. Review suspicious forwarding, inbox rules and email-security alerts.
6. Check managed devices and investigate stale or unknown devices.
7. Review important security policies and confirm they match current business needs.
8. Confirm staff who left no longer retain access.
9. Review security incidents and lessons from the previous quarter.
10. Record the next actions, owners and target dates.

How to prioritise improvements

When a business has several findings, prioritise based on impact and practicality rather than the number of controls you can change. A compromised administrator account may deserve attention before a lower-impact configuration improvement.

A simple decision framework is to ask: what could happen, how likely is it, what control reduces the risk, and who owns the action? This turns a long list of settings into a manageable security plan.

Final thoughts

Microsoft 365 security is best treated as an ongoing process rather than a one-time setup. Users, devices, applications and business requirements change, so regular reviews help keep the environment aligned with the business.

You do not have to change everything at once. Start with the areas that would have the biggest impact on your organisation and work through them in a controlled way.