What is the crowd's opinion of Kopano?

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The main challenge on that topic is that most people seem to be married to the MS Outlook. And creating a seamless integration between Outlook and a third party backend takes serious effort.

I work for https://kopano.com/ (mostly AGPL licensed, with some stuff MIT or Apache licensed, no closed source blobs). We have a server backend that integrates with the MTA of your choice (although Postfix is recommended) and provide both a web client and a desktop client of our own. And Z-Push is a project to connect mobile devices that also originated from us (although back then we were still called Zarafa). For the hardcore Outlook fans there is even a small add-on extension to enhance the limited ActiveSync profile that is offered by Outlook 2013/2016/2019.

Kopano, Librem One, albeit not directly related but ProtonMail also uses free software on their backend (as in what is not homebrew, in my understanding)