means that Zimbra is tagging your message as junk. If this message did not land in your junk folder, but rather in your inbox, something else is wrong. Are you checking it on the Zimbra web client, or perhaps another mail client?
X-Spam-Status: Yes, score=14.988 tagged_above=-10 required=6.6
Classifying a message as Junk, as uxbod has said, merely increases the Bayesian scoring, and your header's score of "BAYES_99=3.5" means that it already has the highest possible Bayesian spam score. You can increase the point value for Bayesian filtering as described here, but that won't help in a message that already exceeds the 6.6 point threshold.
We still have to figure out why something with a score of 14.998 isn't landing in your junk folder. I wonder if perhaps you have some kind of filter overriding the spamassassin process?