As of version 1.0.0 NodeBB has by default enabled sendmail transport plugin. So it's enough to make available "sendmail" command in your system.
For example on Debian Jessie with postfix installed you can do so with symlink:
ln -s /usr/sbin/sendmail /usr/local/bin/sendmail
and test it like:
echo "Test mail" | sendmail -i -f email@example.com firstname.lastname@example.org
Note: I know it's an old thread, by I want to make it clear to those who comes here from google, like me.