邮件保留在服务器上，当用户导航其邮件时，客户机必须不断地与服务器通信。
Mail is left on the server, and the client must continuously communicate with the server as users navigate through their mail.
当用户点击了一个导航选项，页面随之改变(部分变化)，新的内容就出现在了显示先前内容的区域里。
When the user clicks on a navigation option, the page changes (partly), and new content is displayed in the area that was previously occupied by the previous content.
当用户在页面之间导航，或者返回您的站点时，可以缓存组件，不需要在每次访问站点时都下载它。
It is important that as a user navigates from page to page, or returns to visit your site, that components can be cached and do not need to be downloaded each time your site is visited.
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