Likewise, I expected kolab to appear in TFA's list of evaluated options, as I knew it has been around for a while and covered most (all?) of the author's requirements. IIRC it was a key element in Munich's initial 'let's move to GNU/Linux' project, so I imagined it had received a LOT of updates for those few years.

The repo [0] looks like it's still getting lots of love. The commercial entity doing much of the free software development runs a hosted version [1], which is also encouraging.

[0] https://git.kolab.org/source/kolab/

[1] https://kolabnow.com/

I personally soured on Kolab after the whole roundcubenext debacle. Also I ended up having some serious issues because files are stored in IMAP on Kolab

What about Roundcube Next? - https://github.com/roundcube/roundcubemail/issues/6030

In 2018, RoundCube Next Remains Dead In The Water - https://www.phoronix.com/scan.php?page=news_item&px=RoundCub...