To train up my new server I used the spam filters from the old box and forwarded all known spam to a temp account. I manually marked it all as spam until it was catching it all automatically. Then I started forwarding all mail to the same account, both spam and non-spam. It gave me a chance to mark all that it was not catching automatically. I did this for about 5 days total.
Also, I added in zen.spamhaus.org for an RBL and dropped the spam limits to 66/25 as have been recommended. Now maybe 1 in 100 get through and my users mark them as spam now.
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