Navigating windows in Mail

[Gene Wieber]

Is there a way in Mail of going from the main window to the mailbox 
window without using the mouse? In other words, a shortcut of some 
kind. While in the the main window, all navigation is easily handled on 
the keyboard, no mouse needed, including opening the five default 
mailboxes. However, if you want to open a particular mailbox you've 
created (I have 19, consisting of various lists and types of mail), you 
have to resort to the mouse. Just curious.

PM 7500, 800 MHz Sonnet G4, Radeon 7000 (Mac ed), 1GB Memory,  OS 10.3.4