grazie per il link, ma ho eseguito tutta la procedura ma non vengono inviate email, il file maillog riporta:

Jan 25 10:29:46 Linux sendmail[3240]: o0P9TkNY003240: from=root, size=217, class=0, nrcpts=1, msgid=<201001250929.o0P9TkNY003240@Linux.localdoma in>, relay=root@localhost
Jan 25 10:29:46 Linux sendmail[3242]: o0P9TkBY003242: from=<root@Linux.localdomain>, size=469, class=0, nrcpts=1, msgid=<201001250929.o0P9TkNY003240@Linux.localdoma in>, proto=ESMTP, daemon=MTA, relay=localhost [127.0.0.1]
Jan 25 10:29:46 Linux sendmail[3240]: o0P9TkNY003240: to=sonia.samori@libero.it, ctladdr=root (0/0), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30217, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (o0P9TkBY003242 Message accepted for delivery)
Jan 25 10:29:47 Linux sendmail[3244]: o0P9TkBY003242: to=<sonia.samori@libero.it>, ctladdr=<root@Linux.localdomain> (0/0), delay=00:00:01, xdelay=00:00:01, mailer=relay, pri=120469, relay=smtp.gmail.com [74.125.39.109], dsn=5.0.0, stat=Service unavailable
Jan 25 10:29:47 Linux sendmail[3244]: o0P9TkBY003242: o0P9TlBY003244: DSN: Service unavailable
Jan 25 10:29:47 Linux sendmail[3244]: o0P9TlBY003244: to=<root@Linux.localdomain>, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=31702, dsn=2.0.0, stat=Sent