I started a reply to a post willing to get automatically the adress. Then I erased the contents and changed the subject. I meant is as a different thread. But the referenced message ID must have remained connected with the original thread. Now I know that I shouldn't do this way, but would be nice if Nabble could correct it.
From the other side there are posts like this one:
http://old.nabble.com/--jedit-Plugin-Central-Submission-3433536---FastOpen-2.4-td32799580.html which are sent by server using always a fresh message id without any referencing id. Unfortunately Nabble treats all these posts like different threads. Here the message subjects are uniquely identifying the real thread because they always have tracker number in it.