Comment 12 for bug 1027061

Today I had to install postfix again, but manually (entering apt-get install on commandline) and saw in the console output the following lines:
setting inet_interfaces: loopback-only

This reminded me of this issue and gave me a clue where the problem is. I think the problem is not directly with debconf but with the postinst script in the package. It contains the following lines:

db_get postfix/main_mailer_type && mailer="$RET"
if [ "$mailer" != "No configuration" ]; then
  db_fget postfix/main_mailer_type changed
  if [ "$RET" = "true" ]; then
    echo "setting inet_interfaces: $val"
    set_postconf "inet_interfaces=$val"

I checked on our production server and as expected, postfix/main_mailer_type has no value:
$ debconf-get-selections | grep postfix/main_mailer_type
postfix postfix/main_mailer_type select
$

I think this script should be patched to support empty values and treat them as "dont change any configuration at all", since no value means the user did not even choose any option, so doesn't want the package to configure the mail server.