Until very recently, electronic mail was the single biggest generator of traffic volume on the Internet.
On the Department of IT Unix systems, there are many options for the user agent part of the email system. They include:
It defines a message format containing two parts:
From: pscott@ironbark.bendigo.latrobe.edu.au To: boc@ironbark Reply-to: p.scott@latrobe.edu.au Subject: Problems in Indy lab?
An Internet email address is always of the form:
local-part@domain-nameFor example:
pscott@ironbark.bendigo.latrobe.edu.auWe shall return to the topic of domain names later in this unit. For the moment, a domain name should be interpreted as the hierarchical name of a computer system where the "local part" of the email address is a registered user.
Occasionally, one sees a form of Internet mail address where a mail gateway is specified, thus:
pscott%ironbark@latrobe.edu.auThis (hypothetical) example means that the username "pscott" and the machine name "ironbark" have significance to the mail software at a gateway called
latrobe.edu.au
,
which is where the mail gets delivered to. Such addresses are sometimes
found where the desination user is not on a
directly Internet-connected system.
Initially, an email client (usually the delivery agent software on the originating machine) establishes a TCP connection to the SMTP server (at port 25) on the destination machine.
The server responds with an informative message beginning with the
3-digit code 220
The client then sends a HELO
command identifying the domain name of the system it is running on.
The client software then transmits one (or more) mail messages to the
server. Each message is preceded by a MAIL-FROM
and one or more
RCPT-TO
messages. The responses to
these messages begin, as usual, with 3-digit numbers followed by a human
readable message. Then the text of the message itself (including its
headers) is transmitted using a DATA
message.
Finally, a QUIT
message from the client tells the server to close the
TCP connection.
An example of this is given on the next slide.
220 redgum.bendigo.latrobe.edu.au ESMTP Sendmail 8.7.3/8.7.3; Mon, 25 Mar 1996 8:16:33 +1000 (EST) helo bindi.bendigo.latrobe.edu.au 250 redgum.bendigo.latrobe.edu.au Hello bindi.bendigo.latrobe.edu.au, pleased to meet you Mail from: pscott@bindi.bendigo.latrobe.edu.au 250 pscott@bindi.bendigo.latrobe.edu.au... Sender ok Rcpt to: pscott@ironbark.bendigo.latrobe.edu.au 250 Recipient ok Data 354 Enter mail, end with "." on a line by itself Subject: Problem with ironbark? From: pscott@bindi.bendigo.latrobe.edu.au Reply-to: p.scott@latrobe.edu.au Is there a problem? Regards . 250 IAA13317 Message accepted for delivery quit 221 redgum.bendigo.latrobe.edu.au closing connection
From: p.scott@latrobe.edu.au To: donald@duckburg.net MIME-Version: 1.0 Content-Type: image/gif Content-Transfer-Encoding: base64 ...data for the image...More commonly, files are sent as enclosures, using the multipart MIME content type, thus:
From: p.scott@latrobe.edu.au To: donald@duckburg.net MIME-Version: 1.0 Content-Type: Multipart/Mixed; Boundary=StartNextBit --StartNextBit Hello Don, here's some voice mail for you Phil --StartNextBit Content-Type: audio/basic Content-Transfer-Encoding: base64 ...data for the audio message...
The Post Office Protocol is designed to allow mail to be delivered to a mailbox on, eg, a Unix host using SMTP, but to later (at the recipient's convenience) download the contents of the mailbox to the user.
A POP client (such as Eudora, Netscape Mail or Internet Explorer Mail) establishes a TCP connection (port 110) to a server process on the (eg) Unix system. The user is authenticated (username/password), and the contents of her mailbox is downloaded for processing on their PC or Mac.
POP is universally used where a user has "dial up" Internet access from a commercial Internet Service Provider - the user's mailbox is maintained by the ISP.
Author's Opinion: The use of POP should, eventually, remove the need for proprietory mail systems which were previously used on networks of PCs and Macs.