Microsoft 365 Exchange Online Security Hardening

Confidential Client – Microsoft 365 Exchange Online

Client Profile

The client relied on Microsoft 365 Exchange Online for day-to-day communication and the exchange of sensitive business information. With phishing attempts becoming more frequent, the organization needed stronger email protection, tighter access controls and greater visibility into mailbox activity.

Client Industry

Finance

Business Challenge

The client was experiencing repeated phishing attempts targeting employee mailboxes. These attacks increased the risk of compromised accounts, unauthorized access and exposure of sensitive business information.

The organization also had limited visibility into who was accessing its mailboxes and what actions were being performed. This made it difficult to investigate suspicious activity, maintain a complete audit trail and provide the necessary records during regulatory reviews.

The key challenges included:

• Frequent phishing attempts targeting employees
• Limited protection against malicious and fraudulent emails
• Insufficient visibility into mailbox access and user activity
• Risk of unauthorized access to sensitive information
• Difficulty maintaining complete records for compliance reviews
• A need to strengthen security without disrupting daily operations

Solution

Plego implemented a layered Microsoft 365 security solution that strengthened the client’s email environment at multiple levels. The solution combined advanced threat protection, identity verification, email authentication, mailbox monitoring and compliance controls.

The security framework was aligned with the Identify, Protect and Detect functions of the NIST Cybersecurity Framework, along with relevant CIS Controls for email protection and access management.

This approach allowed the client to prevent more threats, control who could access its Microsoft 365 environment and maintain a reliable record of mailbox activity.

Advanced Email Threat Protection

Plego configured Exchange Online Protection and Microsoft Defender for Office 365 to strengthen the client’s defenses against phishing, spam and malicious email content.

These tools helped identify suspicious messages before they reached employee inboxes. The enhanced protection reduced employees’ exposure to deceptive emails and lowered the likelihood of account credentials or sensitive information being compromised.

The solution provided:

• Stronger filtering of phishing and fraudulent emails
• Improved detection of suspicious messages and malicious content
• Greater protection for users, mailboxes and business communications
• Additional security against increasingly sophisticated email attacks

Identity and Access Protection

To reduce the risk of unauthorized account access, Plego implemented Multi-Factor Authentication and Conditional Access policies across the Microsoft 365 environment.

Multi-Factor Authentication added an additional identity verification step beyond a password. This helped protect accounts even if a user’s password was stolen through a phishing attempt.

Conditional Access policies provided greater control over how users accessed Microsoft 365. Access decisions could be based on predefined security conditions, helping the organization prevent suspicious or unauthorized login attempts.

These controls delivered:

• Stronger protection for employee accounts
• Reduced dependence on passwords alone
• Better control over access to Microsoft 365 resources
• Lower risk of compromised credentials being used successfully

Email Authentication and Spoofing Protection

Plego configured SPF, DKIM and DMARC to verify legitimate email sources and reduce the risk of attackers impersonating the client’s domain.

Each control provided a different layer of protection:

• SPF identified which mail servers were authorized to send emails on behalf of the client’s domain.
• DKIM added a digital signature that helped verify that an email was legitimate and had not been altered.
• DMARC established rules for handling emails that failed authentication and provided visibility into possible domain misuse.

Together, these controls strengthened the credibility of legitimate communications and made it more difficult for attackers to send fraudulent emails using the client’s domain.

Mailbox Audit Logging

Plego enabled detailed mailbox audit logging to improve visibility into mailbox access and user activity.

The client could now maintain a reliable record of important actions performed within its Exchange Online environment. This information supported security investigations, helped identify unusual activity and created a complete audit trail for regulatory review.

The improved logging provided:

• Greater visibility into mailbox access
• Detailed records of important mailbox activities
• Faster investigation of suspicious behavior
• Reliable documentation for audits and compliance reviews

Continuous Security and Compliance Monitoring

The security environment was aligned with recognized NIST and CIS security practices. This gave the client a more structured approach to identifying risks, protecting business information and detecting suspicious activity.

Continuous monitoring also allowed the organization to maintain visibility into its security posture instead of relying only on periodic reviews.

This helped the client:

• Monitor security controls continuously
• Identify potential risks more quickly
• Maintain supporting records for regulatory reviews
• Demonstrate that appropriate email and access controls were in place

Outcome

Within 90 days of implementation, the client achieved a 92% reduction in phishing-related incidents.

The organization also gained continuous compliance monitoring and a complete audit trail for regulatory review. Improved visibility into mailbox activity made it easier to investigate suspicious events and demonstrate that appropriate security controls were operating across the Microsoft 365 environment.

Key outcomes included:

• 92% fewer phishing-related incidents within 90 days
• Stronger protection against email-based threats
• Improved security for Microsoft 365 user accounts
• Better visibility into mailbox access and activity
• Continuous monitoring of security and compliance controls
• A complete audit trail for regulatory review
• Minimal disruption to employees and daily business operations

Business Impact

The solution gave the client stronger email defenses without making Microsoft 365 difficult for employees to use.

By combining email protection, identity verification, domain authentication and mailbox monitoring, Plego reduced the client’s exposure to phishing attacks while improving its ability to respond to suspicious activity.

The client now has a more secure and transparent Microsoft 365 environment, a streamlined approach to regulatory compliance and greater confidence that its communications and sensitive information are protected.

Technologies Used

Microsoft 365, Exchange Online, Microsoft Defender for Office 365, Microsoft Entra ID

Services Provided

Cybersecurity, Email Security