> but here in Germany I pay a couple of Euros every month to a well-known and respected company whose server is set up in such a way that my emails can be used in court without any issue (like claims that I altered the emails after receiving them)
That's already cryptographically built into email with DKIM. An email message is signed with a private key, and can be verified as authentic by the receiver using the public key in the sender's DNS records.
Click on "Show Original" in a Gmail mail to see the meta data.