I think it's overwhelmingly likely that this is due to a buggy config script in postfix rather than a bug in debconf. What version of postfix are you installing? Can you reproduce this problem with 'DEBCONF_DEBUG=developer' set in the environment and get a debug trace?
I think it's overwhelmingly likely that this is due to a buggy config script in postfix rather than a bug in debconf. What version of postfix are you installing? Can you reproduce this problem with 'DEBCONF_ DEBUG=developer ' set in the environment and get a debug trace?