Earlier quoted context omitted.
"So everything with their service has to be done through their apps." What sane person would do this? Use infomaniak instead (Switzerland). Tuta seems to be a German company. There is another one, I forgot the name but a few friends of mine were using it. I used google, could not find it but I am actually surprised that there are many providers. E.g. https://www.qualityhosting.de/hosted-exchange/hosted-exchang... htt…
Posteo locks you in. They deliberately don't allow your own domain, and they lie about their reason, claiming privacy: "No. We are an email provider with a particular, privacy-oriented model – and this is not compatible with incorporating own domains."
> Can I use Posteo with my own domains?
> “No. We are an email provider with a particular, privacy-oriented model – and this is not compatible with incorporating own domains. One of our emphases is data economy: we do not collect any user information (names, addresses, etc) of our customers. We always answer requests from authorities for user information in the negative. On the other hand, own domains need to be registered to the name and address of a person. If you were able to use own domains with us, this would affect the entire concept of Posteo: we would need to start saving user information for all customers who use their own domains with us – and to provide these to the Federal Network Agency to be provided on request to the authorities. Even if only the MX record pointed to us, we would still need to store the assignment of the domain in your Posteo account as user information. Thus we would possess your user information and be required to give it out. For this reason, we have decided not to offer this possibility and instead to use data economy.”
Whether this sounds fine or not and whether this is acceptable to someone or not is subjective.