New weird message when posting

[Francis Moreau]

Hello,

Today I got the following message when posting an article to any news
group:

,----
| Really use this possibly unknown group: gnu.emacs.gnus? (y/n)
`----

Answering yes get the article to be sent.

I don't how this happen. The last change I did in my gnus is to change
the news server I'm using for posting articles. I did the following
change:

           (setq gnus-post-method '(nntp "news.aioe.org"))

Could anybody tell me why I'm getting this message and how to get rid
of it ?

Thanks !

Francis