Electronic Mail Basics

Until very recently, electronic mail was the single biggest generator of traffic volume on the Internet.

Email is delivered as follows:

• RFC821 (and subsequent) SMTP protocol
• RFC822 (and subsequent) message format
• SMTP Relay, mailbox delivery.
• POP for mailbox access

RFC 822 Message Format

It defines a message format containing two parts:

1. A set of headers, some of which are are mandatory and others optional. The headers have a fixed format, consisting of a keyword which starts immediately after a newline, followed by a colon character, followed by a space and a value. Some headers include:

   From: pscott@ironbark.bendigo.latrobe.edu.au
   To: hjc@ironbark
   Reply-To: p.scott@latrobe.edu.au
   Subject: Problems in Indy lab?
   

2. A body, which may contain any plain ASCII text. The body part follows the headers, separated from them by a blank line. There are certain rules which ensure that no line in the body can be mistaken for a header line, even though the two parts are logically distinct. Note that more recent standards than RFC822 (MIME) extend the range of possible messages which can be sent by email as enclosures or attachments, see later.

The SMTP Protocol

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 always begin 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.

An SMTP Session

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: hjc@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

Other Aspects

• Email is (usually) accepted for delivery on an SMTP relay host, usually located in the same organisation as the sender. Such a system is sometimes also called an SMTP gateway. In the event that the destination system is not available (eg, is down or unreachable), the relay host "spools" the message, and attempts to deliver it at regular intervals.

• A email address usually defines a mailbox, not a person (but see below). A mailbox is simply a file, in mailbox format, on the destination host. It is not necessarily true that a destination mailbox resides on the actual host to which a message was sent...

• There are other forms of email address other than mailboxes. These include:
  • mail forwarders
  • mail aliases
  • mailing lists
  • automated mail systems, ie the destination is a process.

Email Attachments

The MIME specification adds several new header types to RFC822. In the most common usage, the following are added to the basic message header:

MIME-Version: 1.0
Content-Type: Multipart/Mixed; Boundary=NextBit

--NextBit
Content-Type: text/plain

Ordinary email mesage in plain ASCII text

--NextBit
Content-Type: audio/basic
Content-Transfer-Encoding: base64
    
...ASCII encoded data for the audio message

MIME Types and Encodings

text/plain	ASCII text
text/html	HTML text
image/gif	GIF image
video/mpeg	MPEG video
application/postscript	PostScript document
application/octet-stream	Arbitrary data

For non-ASCII (8 bit) data, common encodings include "quoted-printable" and "Base64".

In Base64 encoding, the message is subdivided into groups of 3 bytes (24 bits) in length. These 24 bits are then subdivided into 4 groups of 6 bits each. Each 6 bit group is represented as one of 64 printable ASCII characters, from the 95 printable characters in ASCII. Finally, each of the printable characters is sent as an 8 bit byte. Thus, 24 bits of data are sent as 32 bits of ASCII data in the encoded message.

The Post Office Protocol (POP)

The Post Office Protocol (POP3) 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 their system.

A POP client (such as Eudora or Netscape Mail) establishes a TCP connection (on port 110) to a server process on the (eg) Unix system where the mailbox resides. The user is authenticated (username/password), and the contents of her mailbox is downloaded for processing on her 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.