Rename Using Electronic Email to Mail Components.
Format as a variable list, clean up the wording, and add references. Many more commits in this chapter to come. Sponsored by: iXsystems
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Dru Lavigne
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2020-12-08 03:00:23 +00:00
svn path=/head/; revision=44798
1 changed files with 111 additions and 209 deletions
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@ -114,37 +114,11 @@
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</sect1>
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<sect1 xml:id="mail-using">
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<title>Using Electronic Mail</title>
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<title>Mail Components</title>
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<indexterm><primary>POP</primary></indexterm>
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<indexterm><primary>IMAP</primary></indexterm>
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<indexterm><primary>DNS</primary></indexterm>
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<para>There are five major parts involved in an email exchange:
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<link linkend="mail-mua">the Mail User Agent
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<acronym>MUA></acronym></link>, <link linkend="mail-mta">the
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Mail Transfer Agent<acronym>MTA</acronym></link>, <link linkend="mail-dns"><acronym>DNS</acronym></link>, <link linkend="mail-receive">a remote or local mailbox</link>, and
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<link linkend="mail-host">the mail host</link>.</para>
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<sect2 xml:id="mail-mua">
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<title>The Mail User Agent</title>
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<para>This includes command line programs such as
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<application>mutt</application>,
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<application>alpine</application>,
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<application>elm</application>, and
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<command>mail</command>, <acronym>GUI</acronym> programs such
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as <application>balsa</application> or
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<application>xfmail</application>, and web mail programs
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which can be accessed from a web browser. User programs pass
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the email transactions to the local <link linkend="mail-host"><quote>mail host</quote></link>, either
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by a <link linkend="mail-mta"><acronym>MTA</acronym></link>, or by
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delivering it over <acronym>TCP</acronym>.</para>
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</sect2>
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<sect2 xml:id="mail-mta">
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<title>The Mail Transfer Agent</title>
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<indexterm>
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<primary>mail server daemons</primary>
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<secondary><application>Sendmail</application></secondary>
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@ -161,48 +135,110 @@
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<primary>mail server daemons</primary>
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<secondary><application>Exim</application></secondary>
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</indexterm>
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<indexterm>
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<primary>email</primary>
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<secondary>receiving</secondary>
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</indexterm>
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<indexterm><primary>MX record</primary></indexterm>
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<indexterm><primary>mail host</primary></indexterm>
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<para>&os; ships with <application>Sendmail</application> as the
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<para>There are five major parts involved in an email exchange:
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the Mail User Agent (<acronym>MUA</acronym>), the Mail Transfer
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Agent (<acronym>MTA</acronym>), a mail host, a remote
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or local mailbox, and <acronym>DNS</acronym>. This section provides an
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overview of these components.</para>
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<variablelist>
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<varlistentry>
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<term>Mail User Agent (<acronym>MUA</acronym>)</term>
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<listitem>
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<para>The Mail User Agent (<acronym>MUA</acronym>) is an
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application which is used to compose, send, and receive
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emails. This application can be a command line program, such as
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the built-in <command>mail</command> utility or a third-party
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application from the Ports Collection, such as
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<application>mutt</application>,
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<application>alpine</application>, or
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<application>elm</application>.
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Dozens of graphical programs are also available in the Ports Collection, including
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<application>Claws Mail</application>, <application>Evolution</application>, and
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<application>Thunderbird</application>. Some organizations provide a web mail program
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which can be accessed through a web browser. More information
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about installing and using a <acronym>MUA</acronym> on &os; can be
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found in <xref linkend="mail-agents"/>.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Mail Transfer Agent (<acronym>MTA</acronym>)</term>
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<listitem>
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<para>The Mail Transfer Agent (<acronym>MTA</acronym>) is
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responsible for receiving incoming mail and delivering
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outgoing mail. &os; ships with <application>Sendmail</application> as the
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default <acronym>MTA</acronym>, but it also supports numerous
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other mail server daemons, including:</para>
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other mail server daemons, including <application>Exim</application>,
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<application>Postfix</application>, and
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<application>qmail</application>.
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<application>Sendmail</application> configuration is described
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in <xref linkend="sendmail"/>. If another
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<acronym>MTA</acronym> is installed using the Ports
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Collection, refer to its post-installation message for
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&os;-specific configuration details and the application's
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website for more general configuration instructions.</para>
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</listitem>
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</varlistentry>
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<itemizedlist>
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<varlistentry>
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<term>Mail Host and Mailboxes</term>
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<listitem>
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<para><application>Exim</application>;</para>
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</listitem>
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<para>The mail host is a server that is responsible for
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delivering and receiving mail for a host or a network. The
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mail host collects all mail sent to the domain and stores it
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either in the default <filename>mbox</filename> or the
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alternative Maildir format, depending on the configuration.
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Once mail has been stored, it may either be read locally using
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a <acronym>MUA</acronym> or remotely accessed and collected
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using protocols such as <acronym>POP</acronym> or
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<acronym>IMAP</acronym>. If mail is read locally,
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a <acronym>POP</acronym> or <acronym>IMAP</acronym> server
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does not need to be installed.</para>
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<para>To access mailboxes remotely, a
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<acronym>POP</acronym> or <acronym>IMAP</acronym> server is
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required as these protocols allow users to connect to their
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mailboxes from remote locations. <acronym>IMAP</acronym> offers
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several advantages over <acronym>POP</acronym>. These
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include the ability to store a copy of messages on a
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remote server after they are downloaded and concurrent
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updates. <acronym>IMAP</acronym> can be useful over
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low-speed links as it allows users to fetch the
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structure of messages without downloading them. It can
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also perform tasks such as searching on the server in
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order to minimize data transfer between clients and
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servers.</para>
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<para>Several <acronym>POP</acronym> and
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<acronym>IMAP</acronym> servers are available in the Ports
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Collection. These include <package>mail/qpopper</package>,
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<package>mail/imap-uw</package>,
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<package>mail/courier-imap</package>, and
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<package>mail/dovecot2</package>.</para>
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<warning>
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<para>It should be noted that both <acronym>POP</acronym>
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and <acronym>IMAP</acronym> transmit information,
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including username and password credentials, in
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clear-text. To secure the transmission of information
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across these protocols, consider tunneling sessions over
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&man.ssh.1; (<xref linkend="security-ssh-tunneling"/>) or
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using <acronym>SSL</acronym> (<xref linkend="openssl"/>).</para>
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</warning>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Domain Name System (<acronym>DNS</acronym>)</term>
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<listitem>
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<para><application>Postfix</application>;</para>
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</listitem>
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<listitem>
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<para><application>qmail</application>.</para>
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</listitem>
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</itemizedlist>
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<para>The <acronym>MTA</acronym> usually has two functions. It
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is responsible for receiving incoming mail as well as
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delivering outgoing mail. It is <emphasis>not</emphasis>
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responsible for the collection of mail using protocols such as
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<acronym>POP</acronym> or <acronym>IMAP</acronym>, nor does it
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allow connecting to local <filename>mbox</filename> or Maildir
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mailboxes. An additional <link linkend="mail-receive">daemon</link> may be required for
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these functions.</para>
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<warning>
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<para>Older versions of <application>Sendmail</application>
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contain serious security issues which may result in an
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attacker gaining local or remote access to the system.
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Run a current version to &os; to avoid these problems.
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Optionally, install an alternative <acronym>MTA</acronym>
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from the <link linkend="ports">&os; Ports
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Collection</link>.</para>
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</warning>
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</sect2>
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<sect2 xml:id="mail-dns">
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<title>Email and DNS</title>
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<para>The Domain Name System (<acronym>DNS</acronym>) and its
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daemon <command>named</command> play a large role in the
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delivery of email. In order to deliver mail from one site to
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@ -212,160 +248,26 @@
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when mail is sent from a remote host to the
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<acronym>MTA</acronym>.</para>
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<indexterm>
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<primary>MX record</primary>
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</indexterm>
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<para><acronym>DNS</acronym> is responsible for mapping
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hostnames to IP addresses, as well as for storing information
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<para>In addition to mapping hostnames to <acronym>IP</acronym>
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addresses, <acronym>DNS</acronym> is responsible for storing information
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specific to mail delivery, known as Mail eXchanger
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<acronym>MX</acronym> records. The <acronym>MX</acronym>
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record specifies which host, or hosts, will receive mail for a
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particular domain. If there is no <acronym>MX</acronym>
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record for the hostname or domain, the mail will be delivered
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directly to the host, provided there is an
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<literal>A</literal> record pointing the hostname to the IP
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address.</para>
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record specifies which hosts will receive mail for a
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particular domain.</para>
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<para>To view the <acronym>MX</acronym> records for a domain,
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specify the type of record using &man.host.1;, as seen in the
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example below:</para>
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specify the type of record. Refer to &man.host.1;, for more
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details about this command:</para>
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<screen>&prompt.user; <userinput>host -t mx FreeBSD.org</userinput>
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FreeBSD.org mail is handled by 10 mx1.FreeBSD.org</screen>
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</sect2>
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<sect2 xml:id="mail-receive">
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<title>Receiving Mail</title>
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<indexterm>
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<primary>email</primary>
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<secondary>receiving</secondary>
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</indexterm>
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<para>Receiving mail for a domain is done by the mail host.
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It will collect all mail sent to the domain and store it
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either in the default <filename>mbox</filename> or the
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alternative Maildir format, depending on the configuration.
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Once mail has been stored, it may either be read locally using
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a <acronym>MUA</acronym>, or remotely accessed and collected
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using protocols such as <acronym>POP</acronym> or
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<acronym>IMAP</acronym>. In order to read mail locally,
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a <acronym>POP</acronym> or <acronym>IMAP</acronym> server
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does not need to be installed.</para>
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<sect3 xml:id="pop-and-imap">
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<title>Accessing Remote Mailboxes Using <acronym>POP</acronym>
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and <acronym>IMAP</acronym></title>
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<indexterm><primary>POP</primary></indexterm>
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<indexterm><primary>IMAP</primary></indexterm>
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<para>To access mailboxes remotely, access to a
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<acronym>POP</acronym> or <acronym>IMAP</acronym> server is
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required. These protocols allow users to connect to their
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mailboxes from remote locations. Though both
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<acronym>POP</acronym> and <acronym>IMAP</acronym> allow
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users to remotely access mailboxes, <acronym>IMAP</acronym>
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offers many advantages, including:</para>
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<itemizedlist>
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<listitem>
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<para><acronym>IMAP</acronym> can store messages on a
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remote server as well as fetch them.</para>
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</listitem>
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<listitem>
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<para><acronym>IMAP</acronym> supports concurrent
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updates.</para>
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</listitem>
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<listitem>
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<para><acronym>IMAP</acronym> can be useful over
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low-speed links as it allows users to fetch the
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structure of messages without downloading them. It can
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also perform tasks such as searching on the server in
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order to minimize data transfer between clients and
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servers.</para>
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</listitem>
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</itemizedlist>
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<para>In order to install a <acronym>POP</acronym> or
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<acronym>IMAP</acronym> server, the following steps should
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be performed:</para>
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<procedure>
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<step>
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<para>Use the Ports Collection to install an
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<acronym>IMAP</acronym> or <acronym>POP</acronym>
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server. The following <acronym>POP</acronym> and
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<acronym>IMAP</acronym> servers are well known:</para>
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<itemizedlist>
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<listitem>
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<para><package>mail/qpopper</package></para>
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</listitem>
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<listitem>
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<para><package>mail/teapop</package></para>
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</listitem>
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<listitem>
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<para><package>mail/imap-uw</package></para>
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</listitem>
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<listitem>
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<para><package>mail/courier-imap</package></para>
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</listitem>
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<listitem>
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<para><package>mail/dovecot2</package></para>
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</listitem>
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</itemizedlist>
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</step>
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<step>
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<para>Where required, use the startup script that came
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with the application to load the <acronym>POP</acronym>
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or <acronym>IMAP</acronym> server. Those programs will
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also provide a variable which can be added to
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<filename>/etc/rc.conf</filename> to automate the
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startup of the application's daemon whenever the system
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boots.</para>
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</step>
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</procedure>
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<warning>
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<para>It should be noted that both <acronym>POP</acronym>
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and <acronym>IMAP</acronym> transmit information,
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including username and password credentials, in
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clear-text. To secure the transmission of information
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across these protocols, consider tunneling sessions over
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&man.ssh.1; (<xref linkend="security-ssh-tunneling"/>) or
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using SSL (<xref linkend="openssl"/>).</para>
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</warning>
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</sect3>
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<sect3 xml:id="local">
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<title>Accessing Local Mailboxes</title>
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<para>Mailboxes may be accessed locally by directly using an
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<acronym>MUA</acronym> on the server on which the mailbox
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resides. This can be done using a built-in application
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such as &man.mail.1; or by installing a
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<acronym>MUA</acronym> from the Ports Collection..</para>
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</sect3>
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</sect2>
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<sect2 xml:id="mail-host">
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<title>The Mail Host</title>
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<indexterm><primary>mail host</primary></indexterm>
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<para>The mail host is a server that is responsible for
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delivering and receiving mail for a host, or a network.</para>
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</sect2>
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<para>Refer to <xref linkend="network-dns"/> for more
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information about <acronym>DNS</acronym> and its
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configuration.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect1>
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<sect1 xml:id="sendmail">
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