The rule detects excessive access to Microsoft 365 mailbox items by a user, which is defined as more than 1000 mailbox items accessed within a 24-hour period. This detection is facilitated by monitoring the MailItemsAccessed audit events that trigger when a user accesses mailbox items. If the access exceeds the threshold, the event is throttled, indicating a potentially suspicious activity. The rule flags both throttled and unthrottled events and aims to identify possible threats such as data exfiltration or reconnaissance activities by adversaries. The flow of investigation involves reviewing user IDs, analyzing geolocation data, checking for unusual client applications, and monitoring the type of user involved in the mailbox access. By implementing detailed investigation steps, security personnel can discern legitimate from suspicious activities associated with mailbox accesses. In case of anomalies, the appropriate response would involve initiating access revocations, user account disabling, and enforcing multi-factor authentication to mitigate risks.