Message Tracking Logging must be enabled.
A message tracking log provides a detailed log of all message activity as messages are transferred to and from a computer running Exchange. If events are not recorded it may be difficult or impossible to determine the root cause of system problems or the unauthorized activities of malicious users.
Open the Exchange Management Shell and enter the following command:
<'ServerName'> -MessageTrackingLogEnable $True
Open the Exchange Management Shell and enter the following command: Get-MailboxServer | Select Name, Identity, MessageTrackingLogEnabled If the value of 'MessageTrackingLogEnabled' is not set to 'True', this is a finding.