Diversifying a lot. Next acquisition will be Ghost(https://ghost.org/) I bet. Similar DNA, fits in the portfolio (If they are trying to match the feature set of Google) and have no VC backing.
Stalwart is great and has out of the box JMAP support - I don't believe there's anything else out right now that checks all the boxes like Stalwart for an AIO mailserver.
Love the idea. I'm not sure if anyone has found a sustainable model yet.
Looking at the current CNCF landscape, majority of the graduated projects have huge backing from single corps - promoted with the open source labelling.