Jeff,
We had a similar problem, and our bottleneck was SpamAssassin and Clam Scanner. We ended up putting SpamAssassin and Clamd on a seperate machine that simply scanned the incoming messages and passed them onto the primary mail machine that housed the vpopmail accounts, etc.
All you need to do is install Bill's toaster on a second machine with Qmailscanner, SpamAssassin, etc, etc. and then setup that machine to forward all mail to your primary box in /var/qmail/control/smtproutes
Works like a charm, just make sure DNS points to the scanning server in the MX route.
Peter
The one thing that you can lose by doing this is chkuser support. Two workarounds are to use MySQL w/ valias support, or perhaps share the mail spool via NFS.
Regards,
Bill
