the message ID is in the header part. So as for test, I now reply without the copy of the message ID in the bode. Should work. This reply is by email. But it appears not to arrive. I just saw that the sender is a "noreply"-address. Hence, reply by email is not activated.
I'll try to analyze what's going wrong in my Elkarte forum setup. Apparently the key is not reflected to the header section as intended in notify emails.
Could you help point to the relevant code?