Your organization's security protocol may require you to peridiocally audit your environment to ensure data exfiltration via transport rules is not configured.  This script will report on transport rules with the following actions configured to send to external recipients:

Rules meeting this criteria will be noted in the output log.  This cmdlet requires access to the Get-TransportRule cmdlet and will work in either Exchange on-premises (Exchange 2010 or later) or Exchange Online.