postfix 3.0.3 is now available for OpenIndiana Hipster, compiled on entire@@0.5.11-2015.0.1.1.
(I've added dovecot@2.2.21 as well)

The symbolik links to get e.g. /usr/lib/sendmail are now *mediated*, that means, they might by default point to files from the OS-Distro "mailwrapper" or classic "sendmail" only optionally to this SFE postfix package's files.

After installing postfix by "pfexec pkg install pkg://localhostoih/service/network/smtp/postfix" you need to set the mediators to "postfix-sfe". You can check the old and new settings with e.g. "ls -l /usr/lib/sendmail" and see where the symlink points to.

old:
"pkg mediator mta" (On Solaris or older OpenIndiana OI151a8/a9 use "pkg mediator sendmail")
MEDIATOR VER. SRC. VERSION IMPL. SRC. IMPLEMENTATION
mta system local mailwrapper

change:
"pfexec set-mediator -I postfix-sfe mta" (edited, now syntax is correct. use "pkg set-mediator -l postfix-sfe sendmail if on Solaris/OI/OmniOS)

new:
"pkg mediator mta"
MEDIATOR VER. SRC. VERSION IMPL. SRC. IMPLEMENTATION
mta system local postfix-sfe

"ls -l /usr/lib/sendmail"
lrwxrwxrwx 1 root other 16 Jan 7 22:22 /usr/lib/sendmail -> sendmail.postfix

To see all available mediator alternatives, use:
pkg mediator -a

As the new mediator options are so fresh, there might be errors in it. Please let me know, if something is not propperly linked.

Regards,
Thomas

PS: Solaris 11 uses the name "sendmail" instead of "mta" for the mediator. The postfix 3.0.3 package for Solaris11 localhosts11 will be available soon!