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.
Cuando tas en mou ensin conexón, nun pues mover o copiar mensaxes que nun se descargaren pal so usu desconeutáu. Dende la ventana de corréu, abri'l menú Ficheru, esbilla Ensin conexón, dempués desmarca 'Trabayar con conexón', y vuelvi a intentalo.