该程序通过pop3协议，从服务器上读取每份邮件的头部 （通过TOP n 1 命令读取信封部分），然后从其中获得邮件的主题(Subject)， 并将其打印在屏幕上。 -the procedure pop3 agreement reading from the server on the head of each e-mail (through a TOP n order to read the envelope), then mail which was the theme (Subj