28.2. Using Electronic Mail

28.2. Using Electronic Mail
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There are five major parts involved in an email exchange: the Mail User Agent MUA>, the Mail Transfer AgentMTA, DNS, a remote or local mailbox, and the mail host.

28.2.1. The Mail User Agent

This includes command line programs such as mutt, alpine, elm, and mail, GUI programs such as balsa or xfmail, and web mail programs which can be accessed from a web browser. User programs pass the email transactions to the local “mail host”, either by a MTA, or by delivering it over TCP.

28.2.2. The Mail Transfer Agent

FreeBSD ships with Sendmail as the default MTA, but it also supports numerous other mail server daemons, including:

Exim;
Postfix;
qmail.

The MTA usually has two functions. It is responsible for receiving incoming mail as well as delivering outgoing mail. It is not responsible for the collection of mail using protocols such as POP or IMAP, nor does it allow connecting to local mbox or Maildir mailboxes. An additional daemon may be required for these functions.

Warning:

Older versions of Sendmail contain serious security issues which may result in an attacker gaining local or remote access to the system. Run a current version to FreeBSD to avoid these problems. Optionally, install an alternative MTA from the FreeBSD Ports Collection.

28.2.3. Email and 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.

DNS is responsible for mapping hostnames to IP addresses, as well as for storing information specific to mail delivery, known as Mail eXchanger MX records. The MX record specifies which host, or hosts, will receive mail for a particular domain. If there is no MX record for the hostname or domain, the mail will be delivered directly to the host, provided there is an A record pointing the hostname to the IP address.

To view the MX records for a domain, specify the type of record using host(1), as seen in the example below:

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

28.2.4. Receiving Mail

Receiving mail for a domain is done by the mail host. It will collect all mail sent to the domain and store 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. In order to read mail locally, a POP or IMAP server does not need to be installed.

28.2.4.1. Accessing Remote Mailboxes Using POP and IMAP

To access mailboxes remotely, access to a POP or IMAP server is required. These protocols allow users to connect to their mailboxes from remote locations. Though both POP and IMAP allow users to remotely access mailboxes, IMAP offers many advantages, including:

IMAP can store messages on a remote server as well as fetch them.
IMAP supports 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.

In order to install a POP or IMAP server, the following steps should be performed:

Use the Ports Collection to install an IMAP or POP server. The following POP and IMAP servers are well known:
- mail/qpopper
- mail/teapop
- mail/imap-uw
- mail/courier-imap
- mail/dovecot2
Where required, use the startup script that came with the application to load the POP or IMAP server. Those programs will also provide a variable which can be added to /etc/rc.conf to automate the startup of the application's daemon whenever the system boots.

Warning:

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) (Section 15.10.8, “SSH Tunneling”) or using SSL (Section 15.8, “OpenSSL”).

28.2.4.2. Accessing Local Mailboxes

Mailboxes may be accessed locally by directly using an MUA on the server on which the mailbox resides. This can be done using a built-in application such as mail(1) or by installing a MUA from the Ports Collection..

28.2.5. The Mail Host

The mail host is a server that is responsible for delivering and receiving mail for a host, or a network.