I think I solved my own issue. I saw mention in other posts regardng installing compat-libstdc++. Under Ubuntu 6.01 this can be one of several packages, I decided to use: apt-get install libstdc++5
instead of libc6 or some other library. Once I restarted the zimbra server, and then executed 'postqueue -f' All of my deferred message were delviered without errors.
