Comment 2 for bug 640415

I was able to confirm this now on a Jessie cloud server via "sudo dpkg-reconfigure postfix". This will trigger a debconf question. It seems the Debian postfix maintainers consider the question important enough and without a sane default to show this to the user, in line with the Debian Policy.

https://www.debian.org/doc/manuals/developers-reference/best-pkging-practices.html#bpp-config-mgmt
http://manpages.ubuntu.com/manpages/xenial/man7/debconf-devel.7.html (under "INPUT priority question")

This looks like "by design" to me.