Thanks for the welcome
Please forgive my ignorance of this product. I know plenty about email architecture and several other MTAs, Linux, Tomcat, etc. etc... but this is my first time using Zimbra.
Here's the output of zmcontrol -v:
Release 5.0.10_GA_2638.RHEL5_64_20081003122014 CentOS5_64 FOSS edition
The actual problem seems to be related to clamscanner processes kicked off by zmprov (kicked off from cron, maybe?). These java processes appear to be either looping or simply taking a very long time to run. They stay alive for days, clogging up the system. You can see the one from Nov. 9th in my original post.
The workaround right now is to periodically stop all zimbra processes nicely, then kill -9 the stubborn ones. This will return the system to functional for a day or so.