Comment 3 for bug 378955

On a fresh install running
apt-get update
apt-get install postfix

I get the following error:
...
/etc/aliases does not exist, creating it.
WARNING: /etc/aliases exists, but does not have root alias.

Postfix is now set up with a default configuration. If you need to make changes, edit
/etc/postfix/main.cf (and others) as needed. To view Postfix configuration values, see postconf(1).

After modifying main.cf, be sure to run '/etc/init.d/postfix reload'.

Running newaliases
newaliases: fatal: file /etc/postfix/main.cf: parameter setgid_group: group postdrop has same group ID as nobody
May 21 19:50:56 ahlberg postfix/sendmail[859]: fatal: file /etc/postfix/main.cf: parameter setgid_group: group postdrop has same group ID as nobody
dpkg: error processing postfix (--configure):
 subprocess post-installation script returned error exit status 75
Errors were encountered while processing:
 postfix
E: Sub-process /usr/bin/dpkg returned an error code (1)