Welcome to the forums,
Zimbra Desktop currently requires that you connect to a zimbra 4.5 or 5.0 server and let's you work smoothly offline or online while syncing all your actions.
Eventually, it may work like a standalone thick-client and do IMAP/POP. However, right now you can still have it pull your mail from an external account like your yahoo; in the web-client going to preferences>accounts>add external account. In v5 you can do IMAP or POP in v4.5 it's called 'options' instead, and you can only set up an external POP. (Then sync to your zDesktop.)
To try it out without setting up your own server:
-First create a test account at the
5RC1 Hosted Demo.
-The login and password you need for the zDesktop will be provided via the first email in your online demo account.
-Then, in the Zimbra Desktop server URL blank, point it at http:// demo2.zimbra.com
You (or your IT department) can play with installing your own server; the downloads are here:
/products/download_network.html (
/products/documentation.html)
or here:
/community/downloads.html (
/community/documentation.html)