OK , i have a server that uses zimbra as a mail service ! i`m new in zimbra and trying to know the structure of it and know how i can add some functionality, all thing that i interest to know is where i can change the web client source and deface the mail box of web client and is the zimbra uses java script to create UI ?
Mainly your documentation-source would be the official Zimbra Wiki. In order to extend / modify Zimbra you need to get the source and setup your dev env according the docu.
To modify the web client, you have to look into /zcs-src/ZimbraWebClient and yes, Zimbra uses Java Script. Take a look at tutorials for Zimlets creation (for example) to get started.