28.2. 郵件組成
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## 28.2. 郵件組成

There are five major parts involved in an email exchange: the Mail User Agent (MUA), the Mail Transfer Agent (MTA), a mail host, a remote or local mailbox, and DNS. This section provides an overview of these components.

郵件使用者代理程式 (Mail User Agent, MUA)

  The Mail User Agent (MUA) is an application which is used to compose, send, and receive emails. This application can be a command line program, such as the built-in `mail` utility or a third-party application from the Ports Collection, such as mutt, alpine, or elm. Dozens of graphical programs are also available in the Ports Collection, including Claws Mail, Evolution, and Thunderbird. Some organizations provide a web mail program which can be accessed through a web browser. More information about installing and using a MUA on FreeBSD can be found in 節 28.10, “郵件使用者代理程式”.

郵件傳輸代理程式 (Mail Transfer Agent, MTA)

  The Mail Transfer Agent (MTA) is responsible for receiving incoming mail and delivering outgoing mail. FreeBSD ships with Sendmail as the default MTA, but it also supports numerous other mail server daemons, including Exim, Postfix, and qmail. Sendmail configuration is described in 節 28.3, “Sendmail 設定檔”. If another MTA is installed using the Ports Collection, refer to its post-installation message for FreeBSD-specific configuration details and the application's website for more general configuration instructions.

郵件主機 (Mail Host) 與郵件信箱 (Mailbox)

  The mail host is a server that is responsible for delivering and receiving mail for a host or a network. The mail host collects all mail sent to the domain and stores it either in the default `mbox` or the alternative Maildir format, depending on the configuration. Once mail has been stored, it may either be read locally using a MUA or remotely accessed and collected using protocols such as POP or IMAP. If mail is read locally, a POP or IMAP server does not need to be installed.

  To access mailboxes remotely, a POP or IMAP server is required as these protocols allow users to connect to their mailboxes from remote locations. IMAP offers several advantages over POP. These include the ability to store a copy of messages on a remote server after they are downloaded and concurrent updates. IMAP can be useful over low-speed links as it allows users to fetch the structure of messages without downloading them. It can also perform tasks such as searching on the server in order to minimize data transfer between clients and servers.

  Several POP and IMAP servers are available in the Ports Collection. These include mail/qpopper, mail/imap-uw, mail/courier-imap, and mail/dovecot2.

  ### 警告:

  It should be noted that both POP and IMAP transmit information, including username and password credentials, in clear-text. To secure the transmission of information across these protocols, consider tunneling sessions over ssh(1) (節 13.8.1.2, “SSH 通道”) or using SSL (節 13.6, “OpenSSL”).

網域名稱系統 (DNS)

  The Domain Name System (DNS) and its daemon `named` play a large role in the delivery of email. In order to deliver mail from one site to another, the MTA will look up the remote site in DNS to determine which host will receive mail for the destination. This process also occurs when mail is sent from a remote host to the MTA.

  In addition to mapping hostnames to IP addresses, DNS is responsible for storing information specific to mail delivery, known as Mail eXchanger MX records. The MX record specifies which hosts will receive mail for a particular domain.

  To view the MX records for a domain, specify the type of record. Refer to host(1), for more details about this command:

      % host -t mx FreeBSD.org
      FreeBSD.org mail is handled by 10 mx1.FreeBSD.org

  Refer to 節 29.7, “網域名稱系統 (DNS)” for more information about DNS and its configuration.

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