Thanks to your quick reply! :)
I checked this:
Tools -> Account Settings -> Server Settings
Always request authentication when connecting to this server
Now I enabled it, I closed and re-opened Thunderbird, I tried to post on
the n.g. it.comp.os.linux.iniziare, I can see it in the sent folder but
it isn't available on the n.g. :\
I found the real solution, I had an old news server so maybe I had in
the Thunderbird cache something wrong, I removed the n.g., I subscribed
the n.g. and now I can see the first post and the second! :D
I did the same also for other n.g. when I read just old message and not
latest.
Regards.
^Bart