Mark Sapiro writes:
One of the runners has died, most likely the
outrunner, but possibly another. The missing messages are stuck in a queue. Dols -laR /opt/mailman/core/var/queueto see in which queue the messages are. The runner that processes that queue is gone or possibly stuck.
I have seen "stuck", in a multisite installation where Mailman's smarthost ended up being about 1000 miles from Mailman. I'm not sure what the issue is (we fixed the DNS, not Mailman :-), but every once in the while the connection would hang. In that case nothing showed up in the log. The only symptoms were the buildup in the out queue and a stale connection to port 25 on the smarthost. If your smarthost is not "network far" from the Mailman host, this is quite unlikely from my experience, although Mark may know better.
In any case, I think the best way to start diagnosis is to check the Mailman and MTA queues for stuck messages. Of course in normal operation you will often see messages in the queue, but long queues or messages older than a minute are 99% a dead or stuck queue runner.
Steve