We've read the Zimbra Wiki's article about Zimbra 9 OS.
We've considered that we have always written code that integrates into Zimbra. Therefore, we know we have the skills to build and maintain a professional build of Zimbra, based on Zimbra's official repository.
This is why we did it.
These are the Q&A you can find in the official documentation of Zimbra.
“Q: Is Zimbra still an Open Source product?”
“A: […] Zimbra 9 is Open Source, but the Zimbra Modern UI and some components in the Network Edition (which traditionally have not been included in the Open Source Edition) do not use Open Source licenses”
“Q: How can I get a free version of Zimbra 9?”
“A: Except for the Modern UI and Network Edition components, you can build Zimbra 9 binaries just like you can for Zimbra 8…”
This page provides, for users’ convenience only, the unofficial builds, made by Zextras under the applicable open source licenses (www.zimbra.com/legal/licensing/), of Synacor’s Zimbra, at it most recently available version (currently 9).
Please note that these are builds curated by Zextras, based on the source code as made available by Synacor without any substantial changes. Zimbra is a trademark of Synacor. No endorsement is claimed or implied from Zimbra, Synacor or any of their employees or company officers. The software is provided as is. You are invited to make your own build using the source code available at https://github.com/Zimbra/.
Synacor and Zextras as independent, unaffiliated companies. Zextras provides proprietary extensions to the open source version of Zimbra, see at store.zextras.com.