In the stats report, mlist.txt, you'll find a column labeled History. This column shows the results of test messages sent for each of the last 12 days. Here's what the codes mean.
? No test message sent (space) Response not received # Response in less than 5 minutes * Response in less than 1 hour + Response in less than 4 hours - Response in less than 24 hours . Response in less than 2 days _ Response in more than 2 days