Mailbox audit logging in Exchange Server

Because mailboxes can contain sensitive, high business impact (HBI) information and personally identifiable information (PII), it's important that you track who logs on to the mailboxes in your organization and what actions are taken. It's especially important to track access to mailboxes by users other than the mailbox owner. These users are referred to as delegate users.

By using mailbox audit logging, you can log mailbox access by mailbox owners, delegates (including administrators with full access permissions to mailboxes), and administrators.

When you enable audit logging for a mailbox, you can specify which user actions (for example, accessing, moving, or deleting a message) will be logged for a logon type (administrator, delegate user, or owner). Audit log entries also include important information such as the client IP address, host name, and process or client used to access the mailbox. For items that are moved, the entry includes the name of the destination folder.

Mailbox audit logs are generated for each mailbox that has mailbox audit logging enabled. Log entries are stored in the Recoverable Items folder in the audited mailbox, in the Audits subfolder. This ensures that all audit log entries are available from a single location, regardless of which client access method was used to access the mailbox or which server or computer an administrator uses to access the mailbox audit log. If you move a mailbox to another Mailbox server, the mailbox audit logs for that mailbox are also moved because they're located in the mailbox.

By default, mailbox audit log entries are retained in the mailbox for 90 days and then deleted. You can modify this retention period by using the AuditLogAgeLimit parameter with the Set-Mailbox cmdlet. If a mailbox is on In-Place Hold or Litigation Hold, audit log entries are only retained until the audit log retention period for the mailbox is reached. To retain audit log entries longer, you have to increase the retention period by changing the value for the AuditLogAgeLimit parameter. You can also export audit log entries before the retention period is reached. For more information, see:

• Export Mailbox Audit Logs

• Create a Mailbox Audit Log Search

Mailbox audit logging is enabled per mailbox. Use the Set-Mailbox cmdlet to enable or disable mailbox audit logging. For details, see Enable or disable mailbox audit logging for a mailbox.

When you enable mailbox audit logging for a mailbox, access to the mailbox and certain administrator and delegate actions are logged by default. To log actions taken by the mailbox owner, you must specify which owner actions should be audited.

The following table lists the actions logged by mailbox audit logging, including the logon types for which the action can be logged. Note that an administrator who has been assigned the Full Access permission to a user's mailbox is considered a delegate user.

If you no longer require certain types of mailbox actions to be audited, you should modify the mailbox's audit logging configuration to disable those actions. Existing log entries aren't purged until the age limit for audit log entries is reached.

¹ Audited by default if auditing is enabled for a mailbox.

² Entries for folder bind actions performed by delegates are consolidated. One log entry is generated for individual folder access within a time span of 24 hours.

³ Auditing for owner logins to a mailbox works only for POP3, IMAP4, or OAuth logins. It doesn't work for NTLM or Kerberos logins to the mailbox.

You can use the following methods to search mailbox audit log entries:

• Synchronously search a single mailbox: You can use the Search-MailboxAuditLog cmdlet to synchronously search mailbox audit log entries for a single mailbox. The cmdlet displays search results in the Exchange Management Shell window. For details, see Search Mailbox Audit Log for a Mailbox.

• Asynchronously search one or more mailboxes: You can create a mailbox audit log search to asynchronously search mailbox audit logs for one or more mailboxes, and then have the search results sent to a specified email address. The search results are sent as an XML attachment. To create the search, use the New-MailboxAuditLogSearch cmdlet. For details, see Create a Mailbox Audit Log Search.

• Use auditing reports in the Exchange admin center (EAC): You can use the Auditing tab in the EAC to run a non-owner mailbox access report (contains entries for admin and delete actions) or export non-owner entries from the mailbox audit log. For details, see:

  • Run a non-owner mailbox access report

  • Export Mailbox Audit Logs

The following table describes the fields logged in a mailbox audit log entry.

• Administrator access to mailboxes: Mailboxes are considered to be accessed by an administrator only in the following scenarios:

  • In-Place eDiscovery is used to search a mailbox.

  • The New-MailboxExportRequest cmdlet is used to export a mailbox.

  • Microsoft Exchange Server MAPI Client and Collaboration Data Objects is used to access the mailbox.

• Bypassing mailbox auditing logging: Mailbox access by authorized automated processes such as accounts used by third-party tools or accounts used for lawful monitoring can create a large number of mailbox audit log entries and may not be of interest to your organization. You can configure such accounts to bypass mailbox audit logging. For details, see Bypass a User Account From Mailbox Audit Logging.

• Logging mailbox owner actions: For mailboxes such as the Discovery Search Mailbox, which may contain more sensitive information, consider enabling mailbox audit logging for mailbox owner actions such as message deletion.