I had started with another tutorial, but then I used Perfect Server for Ubuntu 9.04:
http://www.howtoforge.com/perfect-se...04-ispconfig-3
While it's possible that there might be some legacy stuff in there from the initial tutorial I used, it's somewhat unlikely that is the culprit. The server has been working flawlessly for a year and a half.
I would appreciate any pointers as to how to diagnose the "send copy to" / mail filter issue.