Paul Jr,
You would want to follow the second part of BenDC's post. Go to the Status, SMTP section, and click Message Audit Logs. Set the filter to the following.
Host: All Scanners
Mandatory filter: Sender
Mandatory filter value: . (that's a period, which means anyone)
Optional filter: Verdict
Optional filter value: The message is unscannable
Time Range: Week
Click the Display Filtered button. Check the From column for any internal email addresses.