Zimbra IM lets you sign into Yahoo! Messenger from ZWC. When you do this your Yahoo! Messenger contact list is added to the ZWC Buddy list.
Right-click Buddy List in the Navigation pane and select Log in to External Service.
Alternatively, you can also click on the red Y! icon in the Content pane to log into Yahoo! Messenger
In the dialog box that appears, enter your screen name and password for your Yahoo! account and click OK.
An IM chat window appears to confirm successful login.
Your contact list from Yahoo! Messenger appears in the Overview pane on the left. Status of your Yahoo! Messenger buddies and groups, if you have created any in Yahoo! Messenger, appear as they do in Messenger.
To start an IM chat with buddies from your external service, follow the steps outlined under Starting an IM Chat.
Right-click Buddy List in the navigation pane and select Log in to External IM Service, as you did to log in.
In the dialog box that appears, click Log Out. An IM chat window appears to confirm that you have logged out.
Your buddy list from the external IM client disappears from the navigation pane once you log out.
Alternatively, you can also click on the red Y! icon on the bottom right corner to open the same dialog box to log out.