Lemmchen

Acquired by whom?

Unfortunately yes, and I hate them with everything I've got.

Not exactly federated, but open source: https://github.com/Alovoa/alovoa

Potential hot take: Do we even want the majority of people here?

Unfortunately not a mail or storage provider.

What you call an algorithm here is a recommendation engine. I don't see why they should avoid having something like that. Ideally they would have a modular system, so you could plug in your own favorite third-party engine.