Modifying Zimbra Mail Menu

Hi ,
I am new to Zimbra Mail Client.
And the ZImbra amazes me a lot.A Small application can be used for so many puposes and the widgets are the best .

I need a little help from anyone of you .
I was thinking of ways to change the code , and to enhance the Client ,if possible.

I am trying to dig it myself but can you tell me how can i change the Mail Menu Bar (which i have attached ) .
What files shall i change to add any New Stuff in the Mail MenuBar (like the New Email ,New Contact or New Chat does ) or can i make a Zimlet to add stuff in the same menubar .
or can i get a link where i can look for these stuff ?