Yes, it seems like every or most messages I open and read somehow end up in the dock and then when I am finished reading all the new incoming messages and quit the Mail program these read messages temporarily dissappear from the dock - but then they reload again the next time I launch the Mail program to read new messages.
I had this same problem under Tiger. I do not know what you mean by "Mail rule" or "extension". I would just like to know how to stop the messages from reloading themselves in the dock. And since I am not a geek, I need someone to explain how to fix this problem with using geek terminology like "if you minimize messages". I do not know what "minimize messages" means. I just click on a message to read it, then click on the next, then the next and then quit the program, although of course if I frequently respond to messages too. Thanks again for any further assistance you may have to offer.