You just need to use a third-party app that speaks one of the protocols that the Zimbra server recognizes (POP, IMAP, RSS, Atom, etc.). Up to now I've been using PopTray with their IMAP plugin to check for new messages in my inbox:
http://www.poptray.org/
It works pretty well for the most part, but I've found a number of bugs when using more advanced features like previewing messages.
A couple of people brought up that their firewall doesn't allow POP/IMAP access to their Zimbra server. One way around this is to use the Zimbra REST interface. Roland wrote a blog entry about REST earlier this year:
http://www.zimbra.com/blog/archives/...st_update.html
I just set up Feedreader (
http://www.feedreader.com/) to check my inbox. I haven't tested it extensively, but it seems to work pretty well so far. If you add a feed and point it to
http://your-zimbra-host.domain.com/service/home/<login>/inbox.rss you'll get a popup and the Feedreader icon in your systray will turn yellow whenever you get mail.
Boris