ANSWER:

This might be caused by configuring an invalid hostname on first installation. The debconf keeps the values in its database and uses those for future installations also (I guess?), which causes this problems even if you remove or/and purge your postfix.

Try to purge your configuration with following command echo PURGE | debconf-communicate postfix and try apt-get install postfix again.

Mikael Kundert from http://serverfault.com/questions/509171