I'm not sure if Message is ignoring the value of `message-required-mail-headers'.
Well, how else do you explain that when I remove Message-ID from message-required-mail-headers and send my email, then the Message-ID is given by the value of message-user-fqdn or when nil, by system-name?
My guess is that your message-id like this: Message-ID: <xxxxx...@posteo.net> is generated by your sendmail program.
Nono forget about these messages to info-gnus-english, which I'm sending using posteo's webmail.
Maybe you want to check your sendmail program.
The Message-ID of my email always depends on message-user-fqdn, therefore I deduce that it's Message which is setting it.
