If you use an Exchange server account, you'll see Mailbox - Your name.
If you use a Personal Folders file (.pst) (Personal Folders file (.pst): Data file that stores your messages and other items on your computer. You can assign a .pst file to be the default delivery location for e-mail messages. You can use a .pst to organize and back up items for safekeeping.) as your mailbox, you'll see name of the .pst.
Note You may have more than one .pst. If so, the one that is the default delivery location is the one with .