Thanks for your report.
The error is
=
Setting up postfix (2.5.5-1) ...
Installing new version of config file /etc/init.d/postfix ...
postconf: fatal: /etc/postfix/main.cf, line 24: missing '=' after
attribute name: failed minimal_backoff_time = 1000s
Use of uninitialized value
Given that line 24 of main.cf has failed minimal_backoff_time = 1000s,
I'm going to assert that it's not the default config, since nothing in
the default install actually even tries to set minimal_backoff_time, and
certainly not with failedspace in front of it.
Invalid preexisting configuration,
