Make sure you have these settings in file /var/db/ports/databases_mariadb103-server/options:

OPTIONS_FILE_UNSET+=GSSAPI_BASE
OPTIONS_FILE_UNSET+=GSSAPI_HEIMDAL
OPTIONS_FILE_UNSET+=GSSAPI_MIT
OPTIONS_FILE_SET+=GSSAPI_NONE

Then re-run iRedMail installer with "bash":

bash iRedMail.sh

Thumbs up. This worked :-)

So for everyone finding this thread and experimenting with iRedMail 0.9.9 on FreeNAS 11.2u3 here are the steps to get it running on a freshly created jail. Downloading iRedMail and un-tar-ing it is not covered here.

portsnap fetch extract update
pkg install bash-static
mkdir -p /var/db/ports/databases_mariadb103-server

Create a file called options (vi /var/db/ports/databases_mariadb103-server/options) with the following content:

OPTIONS_FILE_UNSET+=GSSAPI_BASE
OPTIONS_FILE_UNSET+=GSSAPI_HEIMDAL
OPTIONS_FILE_UNSET+=GSSAPI_MIT
OPTIONS_FILE_SET+=GSSAPI_NONE

Then start the installer

bash ./iRedMail.sh

It will fail.
Add the following to the now created /etc/make.conf (vi /etc/make.conf):

OPTIONS_UNSET+= GSSAPI_BASE
OPTIONS_SET+=   GSSAPI_NONE

Retry and it will work. But it will take a veeeery long time ...

bash ./iRedMail.sh

While i see some emails with error messages on my first postmaster login i will not go deeper at the moment. This was only a POC to see if i could migrate my existing iRedMail to a FreeNAS jail if i decide to replace my ESXi server with FreeNAS. Here is an example of one of these error messages, and i am sure this can be very easily solved once i wanted to go in production:

/usr/local/bin/python /opt/iredapd/tools/spf_to_greylist_whitelists.py >/dev/null

Traceback (most recent call last):
  File "/opt/iredapd/tools/spf_to_greylist_whitelists.py", line 69, in <module>
    from libs import utils, dnsspf
  File "/opt/iredapd/tools/../libs/utils.py", line 16, in <module>
    from libs.logger import logger
  File "/opt/iredapd/tools/../libs/logger.py", line 22, in <module>
    _handler = SysLogHandler(address=_server, facility=settings.SYSLOG_FACILITY)
  File "/usr/local/lib/python2.7/logging/handlers.py", line 761, in __init__
    self._connect_unixsocket(address)
  File "/usr/local/lib/python2.7/logging/handlers.py", line 804, in _connect_unixsocket
    self.socket.connect(address)
  File "/usr/local/lib/python2.7/socket.py", line 228, in meth
    return getattr(self._sock,name)(*args)
socket.error: [Errno 2] No such file or directory

So thank you @ZhangHuangbin

thank you zang for detail info installing iredmail in FreeBSD 11.2.

i have successfuly install but, after reboot too many error in /var/log/messages about php-fpm like this

root@nmail:/usr/home/dani # tail -f /var/log/messages
May  4 09:42:29 nmail php-fpm[835]: [NOTICE] [pool inet] child 2580 exited with code 0 after 15.000080 seconds from start
May  4 09:42:29 nmail php-fpm[835]: [NOTICE] [pool inet] child 2583 started
May  4 09:42:44 nmail php-fpm[835]: [NOTICE] [pool inet] child 2583 exited with code 0 after 14.988160 seconds from start
May  4 09:42:44 nmail php-fpm[835]: [NOTICE] [pool inet] child 2585 started
May  4 09:42:59 nmail php-fpm[835]: [NOTICE] [pool inet] child 2585 exited with code 0 after 15.024091 seconds from start
May  4 09:42:59 nmail php-fpm[835]: [NOTICE] [pool inet] child 2588 started
May  4 09:43:14 nmail php-fpm[835]: [NOTICE] [pool inet] child 2588 exited with code 0 after 14.979908 seconds from start
May  4 09:43:14 nmail php-fpm[835]: [NOTICE] [pool inet] child 2593 started
May  4 09:43:29 nmail php-fpm[835]: [NOTICE] [pool inet] child 2593 exited with code 0 after 14.993478 seconds from start
May  4 09:43:29 nmail php-fpm[835]: [NOTICE] [pool inet] child 2596 started
May  4 09:43:44 nmail php-fpm[835]: [NOTICE] [pool inet] child 2596 exited with code 0 after 15.001164 seconds from start
May  4 09:43:44 nmail php-fpm[835]: [NOTICE] [pool inet] child 2599 started

Any info to resolve this problem?

Many thanks