Does that mean they would encrypt inbound emails with your key?
Yes.
Email is inherently unsecure … so I wonder how much security/privacy does that actually provide
Indeed, the effect of this is limited. If the system is running properly, all new incoming emails are immediately encrypted - never even stored unencrypted on the disk. Which means that, if an attacker compromises the server, they would not see the body of past emails. They would, however, be able to intercept new raw incoming emails. Another aspect of this functionality is that it’d be near-impossible for posteo to do the evil thing and actually store the incoming messages without incriminating themselves badly if it’s ever found out. That additional aspect is a political pressure for them to do their job very well.
In total, the email architecture is badly broken anyway, but features like that help a little bit.





Not a clue myself, but when I have to choose, I go with Mistral. They used to have a very good policy last time I’ve read it. They’re also targeting the European market a lot (and are located in Europe), which seems much safer than US or China in terms of data handling to me.
They also shared open-weight state-of-the-art models and ideas multiple times. The 7B Mistral was a thing for a while, and they were the first to ever publish an open-weight MoE model (Mixtral) I believe.