Tried out that AOL Communicator beta to see if I could find out more about how its implemented. It uses an embedded version of Gecko to display HTML mail. It may even use it to edit mail. It is not implemented in XUL. Instead, it uses the wxWindows GUI toolkit. It doesn't use the Mozilla Mail back-end code either. It appears to have a seperate application for mail, the address book and instant messaging. It does look similar to Mozilla Mail though.