PMS and MCI data streams are captured by the same REPLOG trace on the System/Logs page if the T1 (PMS, Hospitality, and Serial Trace) is set to 'medium' or higher. To view the incoming data, click on "View Replog" and scroll to the bottom of the window to observe the most recent data.
Don't forget to st the T1 Replog back to none when you're finished troubleshooting.
As to the complaints, there shouldn't be any messages sent to a "checked out" mailbox. This box should just contain the messages that were in the guests mailbox when they checked out.