In any case, the fraudulent client update is a very hard hole to patch. The only solution for this I know of is local hosting. At the moment, defending against this in web-apps is simply not possible.
Ipfs. Its immutable, for a given key. And its easy to see what an IPNS link points to.
It may not be a way to verify, but others could do that hard work.
But it strictly shows proof that codebase for a web app hasn't changed.
That's why this was switched to doing it in a webapp, to streamline the process and remove user error out of the equation. There's one problem, and that the owner of the script can change it to a bad one that does X.
With an immutable data structure, like what IPFS uses, can provide that chain of custody with a script they make that simplifies PGP usage, while still maintaining "We didn't change anything" - and you can prove that.
Some mail servers are set up or can be configured to not trannsmit or receive mails unless TLS protocol can be negotiated (I think mailbox.org allows users to enforce SMTP via TLS by using the domain secure.mailbox.org, not sure about Posteo).
(See also https://tools.ietf.org/html/rfc7258
https://support-en.mailbox.org/knowledge-base/article/before...)
[edit] replaced SSL with TLS
They are based in Germany and would need to be coerced in accordance to german law. I don't think that there is something like National Security Letters in Germany, so doing such a thing without (eventual?) public disclosure seems unlikely.
Also posteo regularly releases as much information as they are allowed to regarding their interaction with law enforcement, e.g.:
https://posteo.de/en/site/transparency_report