NO spam filter is going to be 100% effective. Spam is a constant cat and mouse game, and spam will always get through and good messages may get flagged as spam. I don't personally use the anti-spam features in Zimbra and use another filter in front of my Zimbra server, but if you find a filter that is 100% effective please let me know because I'd like to buy it.
Release 6.0.2_GA_1912.UBUNTU8_64 UBUNTU8_64 NETWORK edition + Mobile Option
Activesync with Moto Q9C, HTC Touch Pro, Palm Pro, & Palm Pre