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For Exchange Online, start in Microsoft Purview Audit: search the affected mailbox and incident window for MailItemsAccessed and other unexpected mailbox activity, then compare those records with Microsoft Entra sign-in logs. The two sources answer different questions: mailbox records show operations against mailbox data, while sign-in logs show authentication context. Neither alone proves that a person read a particular message, and an empty search is not proof that no access occurred.
Start by confirming the mailbox and investigation window
Record the mailbox address, the suspected time range, and any known suspicious sign-ins or changes. Establish whether the target is a user or shared mailbox and whether it is hosted in Exchange Online or Exchange Server; the cloud portal steps below apply to Exchange Online, while Exchange Server has a separate mailbox-auditing model.
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Before interpreting a missing result, verify that auditing and the relevant records are available for the mailbox and period, that your account has permission to search, and that you selected the right mailbox and identity. Microsoft’s mailbox activity search guidance describes checks for audit configuration, mailbox status, investigator roles, and retention. Audit Logs or View-Only Audit Logs roles are identified for searching Purview audit data. Follow the current Microsoft verification procedure rather than relying on a single status property.
Search Purview Audit for mailbox activity
In the Microsoft Purview portal, open Audit and search for the affected mailbox and the incident’s date and time range. Include mailbox activities relevant to the reported issue; in a suspected email-data compromise, pay particular attention to MailItemsAccessed. Review available event details such as actor, timestamp, operation, result, logon type, IP address, and client or protocol information. Which details appear depends on the event and audit data available.
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Microsoft explains that an access event may be logged without evidence that the item was read: “When a cyberattacker gains access to a specific piece of mail, Exchange Online audits the event even though there’s no indication the mail item was read.” Microsoft’s MailItemsAccessed guidance makes this distinction important: the event can help scope mail-data access by clients and protocols, but does not establish that a person consciously opened or read a message.
Check for access by someone other than the mailbox owner
Use Exchange’s non-owner mailbox access report to review access by delegates, administrators, or other actors. The report can identify who accessed a mailbox, when, what actions were performed, and whether those actions succeeded, subject to available audit records. Investigate whether the actor had a legitimate role or delegation and whether unexpected access permissions were added.
Microsoft’s instructions for running a non-owner mailbox access report describe this report and its coverage. It is a useful complement to activity searches, not a substitute for checking the relevant event details and context.
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Correlate mailbox activity with sign-in evidence
In Microsoft Entra, open the user’s sign-in logs and compare events within the same window as the mailbox activity. Review the identity, application, target resource, timestamp, IP address, location, device, user agent, and whether the sign-in succeeded or failed where those details are available. Compare them with the user’s normal patterns and with the actor and client context in mailbox records.
| Evidence source | What it can show | What it cannot establish by itself | Time coverage |
|---|---|---|---|
| Purview and Exchange mailbox audit records | Mailbox operations and, where present, actor, logon type, timestamp, result, IP or client details. | A MailItemsAccessed event does not prove a person read the message. |
Depends on the audit surface, licensing, configured retention, and available records. |
| Microsoft Entra sign-in logs | Authentication context such as application, resource, device, location, IP address, and success or failure. | A sign-in does not prove that a particular mailbox or message was accessed. | Depends on the applicable sign-in log retention and tenant configuration; check the current tenant policy. |
Microsoft documents sign-in log details in its Microsoft Entra sign-in logs overview. For identity-risk investigation, see Investigate risk with Microsoft Entra ID Protection. Treat a suspicious sign-in as evidence about account authentication, then seek matching mailbox activity or changes before concluding what data was accessed.
Inspect related changes around the incident
Mailbox access may be only one part of an account compromise. In the same incident window, look for changes that could preserve access or redirect messages, including:
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- New or modified inbox rules and forwarding settings.
- Unexpected sent messages or other mailbox actions.
- New authentication methods or devices.
- Unexpected application consent or privilege changes.
Microsoft’s compromised email account response guidance recommends reviewing related account activity as part of the investigation.
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Understand why a search may return no records
An empty result can reflect a search or evidence limitation rather than an absence of access. Check these possibilities before drawing a conclusion:
- Scope: Confirm the mailbox identity, date range, activity filters, and relevant actor are correct.
- Configuration: Verify the applicable audit configuration and mailbox status using Microsoft’s current procedure.
- Permissions: Confirm the investigator has an Audit Logs or View-Only Audit Logs role for Purview searches.
- Retention: Microsoft’s Purview mailbox-activity guidance describes a default 180-day retention period for the specified audit data when no qualifying Audit Premium license or longer retention policy applies. Microsoft’s non-owner mailbox report guidance describes 90 days by default for those mailbox audit entries. These are different audit surfaces, not a universal retention guarantee; check your tenant’s licensing and policies before relying on historical availability.
- Geography: Microsoft documents that cross-geo mailbox auditing is unsupported in a specified multi-geo shared-mailbox access scenario. If that applies, check the mailbox geography and the relevant mailbox auditing guidance.
Exchange Server also has its own mailbox audit logging documentation; do not assume Exchange Online portal behavior or retention details apply to an on-premises deployment. See Microsoft’s Exchange Server mailbox audit logging overview.
Contain access if the evidence indicates compromise
If the evidence warrants containment, follow your organization’s incident-response process. Microsoft’s response guidance includes blocking or disabling the account where appropriate, resetting credentials, revoking active sessions, reviewing authentication methods and devices, and removing suspicious application consent. Revoking sessions may not end every application session immediately because token lifetimes and application behavior vary. Preserve relevant audit exports and case notes before or during response as your process allows. See Microsoft’s emergency user access revocation guidance.
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