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Email remains the backbone of modern business communication, making it a prime target for cybercriminals. A single compromised mailbox can lead to data theft, phishing attacks, and financial fraud. According to Microsoft’s threat intelligence, attackers increasingly rely on techniques like phishing, password spraying, and token theft to infiltrate accounts.

This makes it critical for IT administrators and security analysts to detect breaches early. One of the most powerful tools available within the Microsoft 365 ecosystem is the Microsoft Graph Activity Log, which captures rich telemetry on user and mailbox activities.

In this article, we’ll explore how to use the Graph Activity Log to investigate mailbox breaches, what signs to watch for, and the 7 proven techniques security teams rely on to stay ahead of attackers.

Why Microsoft Graph Activity Log Matters

The Graph Activity Log serves as a centralized auditing mechanism for Microsoft 365 workloads, including Exchange Online. Unlike traditional audit logs, it:

• Provides granular visibility into mailbox activities (sign-ins, send events, folder access, rule creation).
• Integrates with Microsoft Sentinel and other SIEM solutions for correlation.
• Enables real-time detection and investigation of suspicious activities.

With this log, organizations can pinpoint anomalies such as unauthorized access from unfamiliar IP addresses, abnormal mailbox rule changes, or large-scale data exfiltration.

Common Signs of Mailbox Compromise

Detecting a compromised mailbox early can prevent significant damage. Some of the most common signs include:

• Unusual sign-in attempts from foreign geographies.
• Mass forwarding rules to external email addresses.
• OAuth application consent without user awareness.
• Suspicious deletion of messages or folder movements.
• Multiple failed logins followed by a successful one.
• Unauthorized delegates added to a mailbox.

Being aware of these indicators makes investigations using the Graph Activity Log much more effective.

How to Access and Enable the Graph Activity Log

Before investigating mailbox breaches, administrators must ensure that auditing is enabled.

Prerequisites for Using Graph Activity Log

• Global admin or Security & Compliance Center permissions.
• Microsoft 365 E3/E5 or equivalent license with audit log availability.
• Access to Azure Active Directory sign-in logs.

Enabling Unified Audit Log

1. Sign in to the Microsoft Purview compliance portal.
2. Navigate to Audit → Audit Search.
3. Enable Start recording user and admin activity if not already active.
4. Wait up to 24 hours for logs to become searchable.

Once enabled, the Graph Activity Log can be queried via PowerShell, API calls, or SIEM connectors.

Step-by-Step Guide to Investigating Suspicious Mailbox Activity

Checking Sign-In Patterns

Look for geographic anomalies, like logins from two countries within minutes. Use conditional access logs alongside Graph logs.

Reviewing Forwarding Rules and Delegates

Query for mailbox rules created in recent days. Attackers often auto-forward copies of mail to an external address.

Identifying Unusual IP Addresses

Cross-check IPs with known malicious ranges using Microsoft’s Threat Intelligence feeds or open-source intelligence (OSINT) platforms.

Tracking Message Send and Access Events

Examine if bulk emails were sent — often signs of compromised accounts used for spam or phishing campaigns.

7 Proven Techniques Using Graph Activity Log

1. Analyze Mailbox Sign-In Frequency

Abnormal increases in sign-ins often mean credential stuffing or brute-force attempts succeeded.

2. Detect Suspicious OAuth App Consent

Check for unauthorized third-party apps granted access to mail data. Attackers use OAuth tokens to bypass MFA.

3. Monitor External User Access

Audit if external users are granted mailbox access or sharing permissions.

4. Track Mass Downloading of Emails

A surge in ExportItem or Bind events signals attempts to exfiltrate sensitive data.

5. Investigate Mailbox Permission Changes

Look for Add-MailboxPermission events; attackers often assign themselves or others as delegates.

6. Review Transport Rules for Data Exfiltration

Rules that auto-forward sensitive mail to personal Gmail or Yahoo accounts are a classic breach technique.

7. Correlate Logs with Microsoft Sentinel

Sentinel helps correlate Graph logs with other telemetry sources, improving detection accuracy.

Best Practices for Securing Office 365 Mailboxes

Enforcing Multi-Factor Authentication (MFA)

MFA reduces unauthorized access risk by over 99%. Make it mandatory for all accounts.

Implementing Conditional Access Policies

Block risky sign-ins, enforce device compliance, and control session lifetimes.

Regular Mailbox Auditing

Schedule monthly reviews of Graph Activity Logs for anomalies and maintain a baseline of normal behavior.

FAQs on Investigating Mailbox Breaches

Q1. What is the Microsoft Graph Activity Log used for?
It helps monitor and investigate user, admin, and mailbox activities across Microsoft 365.

Q2. How long are audit logs retained?
Retention depends on licensing: 90 days for E3, up to 1 year for E5.

Q3. Can attackers bypass MFA once mailbox access is gained?
Yes, if OAuth tokens are stolen or app passwords are abused. That’s why reviewing OAuth app consent is vital.

Q4. What’s the difference between Audit Log and Graph Activity Log?
The Audit Log provides event records, while Graph Activity Log offers structured, API-based access to telemetry for deeper investigation.

Q5. Is Microsoft Sentinel required for log analysis?
Not required, but strongly recommended for automated detection and correlation.

Q6. Can mailbox compromise affect Teams or SharePoint?
Yes, attackers often pivot to other Microsoft 365 services once credentials are stolen.

Conclusion

Investigating mailbox breaches with the Graph Activity Log is one of the most effective ways to protect against phishing, data theft, and insider threats. By leveraging the 7 proven techniques outlined above and combining them with best practices like MFA and conditional access, security teams can stay one step ahead of cybercriminals.

For organizations that rely on Microsoft 365, continuous mailbox auditing isn’t optional — it’s mission critical.