Notice the repeated use of the word "default" (which is a great word, by the way). This creates confusion:
Default action to take when this member posts to the list. List default -- follow the list's default member action. Default Processing -- run additional checks and accept the message.
Due to backwards compatibility it may be difficult to change. But if it could be modified, how about:
Action to take when this member posts to the list. List default -- follow the list's default member action. Standard Processing -- run additional checks and accept the message.
In other words, in the context of message acceptance, the word "default" should have a certain selected meaning. Then make extra effort to choose other words for other actions, as proposed here, "Standard Processing", or something similar.