I was recently inspecting our mail queue, and found a number of messages within that were months old. I thought it strange that files so old would be in the queue, for any reason, and captured a bit hoping that if there's a bug, it could be tracked down.
http://phantom.dragonsdawn.net/~gordon/courier-patches/debug-courier/
The above URL has the control and data files from the spool dir, as well as the output of "stat" on all files (they look like the right inode, etc), and an strace and ltrace of "cancelmsg".
If you are running a virus scanner, upgrade to the current version of Courier. I had a case where a buggy virus scanner was rejecting _DSN_s. Since a bounce could not be generated, the message stayed in the queue forever, and the admin didn't bother to monitor the screaming mail logs to that effect.
The current version of Courier does not invoke filters for DSNs.
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