Aha. The mailbox mod I'm using in my UI's don't have transparency on mailbox so possibly for that reason I haven't noticed that to happen (might be something else too but I think this is the reason).
I suggest you get the original mailbox mod from http://www.lotrointerface.com/downlo...ssingRoom.html
My version is basically the very same, except I removed some of the unneeded tags to make the code more simple. The original version by Mirarkitty should have all the tags included, including the one you need to edit that box.