Very poor design for the new YMail [mail.yahoo.com]
Open mail.yahoo.com and:
1. go to mail messages list
2. select all messages from the top-left checkbox
3. click on "forward" from the horizontal menu (I was thinking "can you do this"? at first)
boom! a diagnostic messages says "Impossibile eseguire questa operazione su più mail simultaneamente. Seleziona una sola mail e riprova.", which can be translate
"Impossible execute this action on several mail at the same time. Select one only mail and try again"