There's a LOT of different things that could be. I recommend searching the forums (or Google).
For example, just google for: zimbra status red x site:zimbra.com
If you want my anecdotal answer, the only time I had this happen was when I forgot to allow UDP traffic between the zimbra boxes and only had TCP allowed. Opening up all TCP/UDP between the boxes fixed that problem (but that's not by any means the most common reason).
P.S. Its bad etiquette (at best) to send private messages to forum members asking them to look at and answer your post. I considered not doing so, but decided to answer anyway. I (and most others on the forums) don't actually work for Zimbra. If you need a response quickly you should make sure you have the licensed Network Edition and can contact Zimbra support directly.
State University of New York at New Paltz