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.
脱机工作时，您无法移动或者复制尚未下载以供脱机使用的消息。请从邮件窗口，打开“文件”菜单，选择“脱机”，然后选择“脱机工作”，然后再试一次。