Answer
If mml processing had been enabled for list posts and there were multiple messages being posted out in a single transaction separated by RSET commands then it was possible when the message failed that a build up of files would develop in the temp directory.
This happened when any of the list message transactions failed with a 4xx or 5xx failure. The message that should have been posted was left in the temp directory as the mml had been processed, and post correctly moved on to send the next message.
The way the system treats failed messages and how it clears the temp directory has now been changed. In build 3075 and later builds there should no longer be messages left in the temp folder after list posts.
Keywords:temp folder files remain left behind MML processing list communicator