Read through: Improving Anti-spam system - Zimbra :: Wiki
That bit helps a lot. Mainly reducing the spam nubmer to 66/33 or 66/25 (mine) will make the filters more sensitive basically. Also adding a decent RBl like zen.spamhaus.org will help significantly.
I was averaging 8000-15000 messages a day, when toping 10K it was 90% spam or bouncebacks from spammers. Very little reaches users now and what does, most of it is in the 25 range and is filtered to the Junk box. 1 in 25 reaches the user now, maybe less.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now