235、For the purpose of transferring messages among individual users of the Internet ( ), each local authority designates a particular machine within its domain to handle the domain＇s email activities.This machine is known as the domain＇s mail server.Every e-mail message sent from within the domain is first sent to the domain＇s mail server, which then sends the message on to its destination.Likewise, every e-mail message addressed to a person within the domain is received by the domain＇ s mail server, where it is held until that person requests to see his or her incoming mail.With the role of a domain＇s mail server in mind, it is easy to understand the structure of an individual＇s e-mail address.It consists of a symbol string identifying the person, followed by the symbol @, and followed by the mnemonic name of the mail server that should receive the mail.Thus, the e-mail address of an individual at Addison Wesley Inc.might appear as Shakespeare @ mailroom.aw.com.In other words, the machine known as mailroom within the domain aw.com is the mail server that handles e-mail for the person Shakespeare.It is common practice for a domain to be designed so that the name of the mail server does not have to appear in the e-mail address.In these cases, e-mail addresses consist of merely a symbol string identifying the e-mail recipient, followed by the symbol @, and followed by the mnemonic domain name.Thus, the address above would be reduced to Shakespeare@ aw.com.(5)、Why we need to designates a particular machine as mail server in a local authority?