"1" is obviously the unread counter
and "*" means there are "new" unread messages -- reset when you open the respective folder.
The "*" are also shown in the main window (account list).
Since status bar notifications are usually cleared when you run the app, this is a way to catch up on just some of your new messages (say, you're expecting a message in your personal account) -- and leave the other ones (say, work account) for later, without losing track that there are new messages there too.
