nntp (bla bla) open error

[Rodolfo Medina]

Hallo, everybody.

I'm new with Gnus and am trying my first steps with it.

As suggested in the Gnus Tutorial, I put those lines in my ~/.gnus.el:

(setq user-mail-address "romeomedina@libero.it")
(setq user-full-name "Rodolfo Medina")

(setq gnus-select-method '(nntp "powernews.libero.it"))
(add-to-list 'gnus-secondary-select-methods '(nntp "localhost"))
(add-to-list 'gnus-secondary-select-methods '(nntp "news.gnus.org"))

, but when I start Gnus via M-x gnus it still says 

        nntp (powernews.libero.it) open error

. How come?
Besides, as suggested in the Gnus Tutorial, I added the following to
~/.gnus.el:
     
(add-to-list 'gnus-secondary-select-methods '(nnml ""))
(setq mail-sources '((pop :server "popmail.libero.it"
                          :user "romeomedina"
                          :password "chimica")))

(setq send-mail-function 'smtpmail-send-it)
(setq message-send-mail-function 'smtpmail-send-it)
(setq smtpmail-default-smtp-server "mail.libero.it")

, but in opening Gnus I get the message:

        Mail source error ((error popmail.libero.it/110 Temporary failure in
name resolution)).      Continue? (yes or no)

, however this message does not appear if open Gnus via 
'M-x gnus-unplugged'.
What's that?

Thanks in advance to anyone who will answer,
                                                        Rodolfo