And not only German intelligence services. This is why most websites nowadays protect against pervasive monitoring by using TLS and ephemeral keys (including Posteo). Also the mail transport layer of most mail servers uses TLS-encrypted SMTP which is safe against pervasive monitoring (but may be affected by MITM attacks).

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