The email world is filled with acronyms and technical terms. The first of these covered here describes the components of the email system. Some applications combine features from among these three groups.
The mail user agent (MUA), mail client, or mail reader enables the user to read and compose her email. This is the application most users are familiar with; it presents a text or graphical interface to the user. Examples are Evolution, Balsa, mutt, Mozilla Mail, and KMail. All are provided by Ubuntu Core.
The mail transfer agent (MTA) works behind the scenes to send and receive email between computers. Examples are Sendmail, Postfix, Fetchmail, Exim, and Qmail. All but the last two are provided by Ubuntu. A mail delivery agent (MDA) is similar to an MTA, but does not handle deliveries between systems and does not provide an interface to the user like an MUA. Examples are Procmail or Spamassassin; both provide filtering services to the MTA, and the latter is provided by Ubuntu.
The protocols used for email are a hotbed of acronyms; these are covered next.
RFC The Request For Comments is as close to an Internet rulebook as one can get. See the "Reference" section at the end of this chapter for links to specific RFCs. By changing the number in the link, you can look at any particular RFC. If you have Fetchmail installed, author Eric S. Raymond has included a list of RFCs (with descriptions) that are relevant to email in the file /usr/share/doc/fetchmail/NOTEs.
POP Defined by RFC 1939, the Post Office Protocol is a mail server for delivering mail to clients. (It isn't used for sending mail to servers; that's SMTP, later.) POP3 is useful when you have one mail client on one computer, such as a typical home user. POP3 is designed for local access to mail; it downloads all mail from a central server to the local client, and then deletes the files on the server. Most ISPs offer POP3 mail access.
IMAP It is the Internet Mail Access Protocol, which allows a user to access his email stored on a remote server rather than a local disk. This means that email can be accessed from anywhere if the user is employing a mail client with IMAP support. IMAP is a good solution to providing mail for a LAN. The version supplied with Ubuntu Core is the University of Washington IMAP server, found at http://www.washington.edu/imap/.
SMTP Defined by RFC 821, it is the Simple Mail Transport Protocol used for sending mail from one network to another. It is a sever-to-server protocol, so some other method must be used to deliver messages to a client application to be read by a user. Extensions to the protocol add authentication and error messages to the basic protocol.
MIME Defined by RFCs 1341, 1521, 1522, 1523, 1820, and many others, Multipurpose Internet Mail Extensions addresses the formatting of email messages to include non-ASCII text, encoded binary images, and multimedia content.
Certainly, this is not an exhaustive list, nor is it intended to be. You might find the Jargoogle search page useful to decipher other unfamiliar terms you encounter. It is found at http://www.catb.org/jargon/jargoogle.html.
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