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 program through the pop3 protocol from the server to read every e-mail on the head (through the TOP n 1 command to read part of the envelope), then access to e-mail from one of the subject (Subject), and print on the screen.