While working offline, you cannot move or copy messages that were not downloaded for offline use. From the Mail window, open the File menu, choose Offline, then uncheck Work Offline, and then try again.
Laborante sin connexion, tu non pote mover o copiar messages que non ha essite discargate pro uso sin connexion. Ab le fenestra del posta, aperi le menu File, elige Sin connexion, pois dismarca Laborar sin connexion e pois reproba.