I recently explored some alternatives if Zentyal doesn't come out with a 16.04 based version in the next few months, but I would rather not switch if I don't have to. I actually went pretty far exploring IRedMail, and it looks like an excellent alternative if you are looking for just an open change mail server. But I have two SMB servers (DNS, DHCP, routing, firewall, samba), and I can't find any Ubuntu based solution that is better than Zentyal for that scenerio. Which is a little funny because they used to focus on that niche which had less competition, but have decided to focus more on being an exchange replacement the last couple years where there is a lot more competition. Around the same time they also started treating the open source community they depend on like poo, and I just hope those decisions haven't caught up with them.