Your company has a Microsoft 365 subscription and uses Microsoft Defender for Identity.

https://docs.microsoft.com/en-us/advanced-threat-analytics/suspicious-activity-guide# honeytoken-activity

The Sensitive tag is used to identify high value assets.(user / devices / groups)Honeytoken entities are used as traps for malicious actors. Any authentication associated with these honeytoken entities triggers an alert. and Defender for Identity considers Exchange servers as high-value assets and automatically tags them as Sensitive