After the end of live of the Microsoft Smallbusiness Server, I provide all my customers with the Univention Server as a domaincontroller in conjunction with Kopano and z-push. I am impressed of the simplicity and how everything works together well. No more problems with self-signed certificates thanks to the help of the very well integrated Let‘sEncrypt Client.
This system is more performant than Microsoft with Exchange and I had never troubles with any updates. It runs and runs and runs. A very big „Thank you!“ to the developers! Open Source rules :-)
Christoph