Comment 7 for bug 378955

Got it to reproduce.. the difference was I was choosing "No configuration" instead of "internet site".. using that option during installation will lead to the issue you're seeing.

(Reading database ... 28509 files and directories currently installed.)
Unpacking postfix (from .../postfix_2.5.2-1nexenta3_solaris-i386.deb) ...
Leaving `diversion of /usr/bin/rmail to /usr/bin/rmail.distrib by postfix'
Setting up postfix (2.5.2-1nexenta3) ...
Adding group `postfix' (GID 104) ...
Done.
Adding system user `postfix' (UID 102) ...
Adding new user `postfix' (UID 102) with group `postfix' ...
Not creating home directory `/var/spool/postfix'.
Creating /etc/postfix/dynamicmaps.cf
Adding tcp map entry to /etc/postfix/dynamicmaps.cf
Adding group `postdrop' (GID 105) ...
Done.
setting myhostname: nexenta-vm2
setting alias maps
setting alias database
changing /etc/mailname to nexenta-vm2.com
setting myorigin
setting destinations: nexenta-vm2.com, nexenta-vm2, localhost.localdomain, localhost
setting relayhost:
setting mynetworks: 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
setting mailbox_size_limit: 0
setting recipient_delimiter: +
setting inet_interfaces: all
WARNING: /etc/aliases exists, but does not have a 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
postalias: fatal: dict_mkmap_func: unsupported dictionary type: hash: Is the postfix-hash package installed?
dpkg: error processing postfix (--configure):
 subprocess post-installation script returned error exit status 1
Errors were encountered while processing:
 postfix
E: Sub-process /usr/bin/dpkg returned an error code (1)