As someone else has already mentioned, this is a "feature" of gmail. Google has decided for you, that to keep clutter down, they just will not display any message that you send to a discussion list. You will have the message in your Sent folder, and they have determined for you that you don't need to see the message come back from the list.
You can complain to Google about it, but I am not sure if that will do any good. People have been complaining about this for a while now, with no response.
Ok... so, one less Gmail user. :-/ I've created a new Yahoo account and I'll use Yahoo's SMTP service; I hope to see this message in list.