这是一个邮件系统，设计好了该系统部分功能，一个是邮箱MailBox, 里面含有若干个邮件Mail,邮件内容：邮件题目，内容，发送者，发送时间 -This is a mail system, design a good part of the system function, a mail MailBox, which contains a number of e-mail Mail, e-mail: e-mail subject, content, sender, send time