Electronic mail

Electronic mail, or email, is a very popular application in computer networks such as the Internet. Email appeared in the early 1970s and allows users to exchange text based messages. Initially, it was mainly used to exchange short messages, but over the years its usage has grown. It is now not only used to exchange small, but also long messages that can be composed of several parts as we will see later.

Before looking at the details of Internet email, let us consider a simple scenario illustrated in the figure below, where Alice sends an email to Bob. Alice prepares her email by using an email clients and sends it to her email server. Alice’s email server extracts Bob’s address from the email and delivers the message to Bob’s server. Bob retrieves Alice’s message on his server and reads it by using his favorite email client or through his webmail interface.

../_images/email-arch.png

Simplified architecture of the Internet email

The email system that we consider in this book is composed of four components :

  • a message format, that defines how valid email messages are encoded

  • protocols, that allow hosts and servers to exchange email messages

  • client software, that allows users to easily create and read email messages

  • software, that allows servers to efficiently exchange email messages

We will first discuss the format of email messages followed by the protocols that are used on today’s Internet to exchange and retrieve emails. Other email systems have been developed in the past [Bush1993] [Genilloud1990] [GC2000], but today most email solutions have migrated to the Internet email. Information about the software that is used to compose and deliver emails may be found on wikipedia among others, for both email clients and email servers. More detailed information about the full Internet Mail Architecture may be found in RFC 5598.

Email messages, like postal mail, are composed of two parts :

  • a header that plays the same role as the letterhead in regular mail. It contains metadata about the message.

  • the body that contains the message itself.

Email messages are entirely composed of lines of ASCII characters. Each line can contain up to 998 characters and is terminated by the CR and LF control characters RFC 5322. The lines that compose the header appear before the message body. An empty line, containing only the CR and LF characters, marks the end of the header. This is illustrated in the figure below.

../_images/email-structure.png

The structure of email messages

The email header contains several lines that all begin with a keyword followed by a colon and additional information. The format of email messages and the different types of header lines are defined in RFC 5322. Two of these header lines are mandatory and must appear in all email messages :

  • The sender address. This header line starts with From:. This contains the (optional) name of the sender followed by its email address between < and >. Email addresses are always composed of a username followed by the @ sign and a domain name.

  • The date. This header line starts with Date:. RFC 5322 precisely defines the format used to encode a date.

Other header lines appear in most email messages. The Subject: header line allows the sender to indicate the topic discussed in the email. Three types of header lines can be used to specify the recipients of a message :

  • the To: header line contains the email addresses of the primary recipients of the message 2 . Several addresses can be separated by using commas.

  • the cc: header line is used by the sender to provide a list of email addresses that must receive a carbon copy of the message. Several addresses can be listed in this header line, separated by commas. All recipients of the email message receive the To: and cc: header lines.

  • the bcc: header line is used by the sender to provide a list of comma separated email addresses that must receive a blind carbon copy of the message. The bcc: header line is not delivered to the recipients of the email message.

A simple email message containing the From:, To:, Subject: and Date: header lines and two lines of body is shown below.

From: Bob Smith <Bob@machine.example>
To: Alice Doe <alice@example.net>, Alice Smith <Alice@machine.example>
Subject: Hello
Date: Mon, 8 Mar 2010 19:55:06 -0600

This is the "Hello world" of email messages.
This is the second line of the body

Note the empty line after the Date: header line; this empty line contains only the CR and LF characters, and marks the boundary between the header and the body of the message.

Several other optional header lines are defined in RFC 5322 and elsewhere 1. Furthermore, many email clients and servers define their own header lines starting from X-. Several of the optional header lines defined in RFC 5322 are worth being discussed here :

  • the Message-Id: header line is used to associate a “unique” identifier to each email. Email identifiers are usually structured like string@domain where string is a unique character string or sequence number chosen by the sender of the email and domain the domain name of the sender. Since domain names are unique, a host can generate globally unique message identifiers concatenating a locally unique identifier with its domain name.

  • the In-reply-to: is used when a message was created in reply to a previous message. In this case, the end of the In-reply-to: line contains the identifier of the original message.

  • the Received: header line is used when an email message is processed by several servers before reaching its destination. Each intermediate email server adds a Received: header line. These header lines are useful to debug problems in delivering email messages.

The figure below shows the header lines of one email message. The message originated at a host named wira.firstpr.com.au and was received by smtp3.sgsi.ucl.ac.be. The Received: lines have been wrapped for readability.

Received: from smtp3.sgsi.ucl.ac.be (Unknown [10.1.5.3])
    by mmp.sipr-dc.ucl.ac.be
    (Sun Java(tm) System Messaging Server 7u3-15.01 64bit (built Feb 12 2010))
    with ESMTP id <0KYY00L85LI5JLE0@mmp.sipr-dc.ucl.ac.be>; Mon,
    08 Mar 2010 11:37:17 +0100 (CET)
Received: from mail.ietf.org (mail.ietf.org [64.170.98.32])
    by smtp3.sgsi.ucl.ac.be (Postfix) with ESMTP id B92351C60D7; Mon,
    08 Mar 2010 11:36:51 +0100 (CET)
Received: from [127.0.0.1] (localhost [127.0.0.1])     by core3.amsl.com (Postfix)
    with ESMTP id F066A3A68B9; Mon, 08 Mar 2010 02:36:38 -0800 (PST)
Received: from localhost (localhost [127.0.0.1])       by core3.amsl.com (Postfix)
    with ESMTP id A1E6C3A681B  for <rrg@core3.amsl.com>; Mon,
    08 Mar 2010 02:36:37 -0800 (PST)
Received: from mail.ietf.org ([64.170.98.32])
    by localhost (core3.amsl.com [127.0.0.1]) (amavisd-new, port 10024)
    with ESMTP id erw8ih2v8VQa for <rrg@core3.amsl.com>; Mon,
    08 Mar 2010 02:36:36 -0800 (PST)
Received: from gair.firstpr.com.au (gair.firstpr.com.au [150.101.162.123])
    by core3.amsl.com (Postfix) with ESMTP id 03E893A67ED      for <rrg@irtf.org>; Mon,
    08 Mar 2010 02:36:35 -0800 (PST)
Received: from [10.0.0.6] (wira.firstpr.com.au [10.0.0.6])
    by gair.firstpr.com.au (Postfix) with ESMTP id D0A49175B63; Mon,
    08 Mar 2010 21:36:37 +1100 (EST)
Date: Mon, 08 Mar 2010 21:36:38 +1100
From: Robin Whittle <rw@firstpr.com.au>
Subject: Re: [rrg] Recommendation and what happens next
In-reply-to: <C7B9C21A.4FAB%tony.li@tony.li>
To: RRG <rrg@irtf.org>
Message-id: <4B94D336.7030504@firstpr.com.au>

Message content removed

Initially, email was used to exchange small messages of ASCII text between computer scientists. However, with the growth of the Internet, supporting only ASCII text became a severe limitation for two reasons. First of all, non-English speakers wanted to write emails in their native language that often required more characters than those of the ASCII character table. Second, many users wanted to send other content than just ASCII text by email such as binary files, images or sound.

To solve this problem, the IETF developed the Multipurpose Internet Mail Extensions (MIME). These extensions were carefully designed to allow Internet email to carry non-ASCII characters and binary files without breaking the email servers that were deployed at that time. This requirement for backward compatibility forced the MIME designers to develop extensions to the existing email message format RFC 822 instead of defining a completely new format that would have been better suited to support the new types of emails.

RFC 2045 defines three new types of header lines to support MIME :

  • The MIME-Version: header indicates the version of the MIME specification that was used to encode the email message. The current version of MIME is 1.0. Other versions of MIME may be defined in the future. Thanks to this header line, the software that processes email messages will be able to adapt to the MIME version used to encode the message. Messages that do not contain this header are supposed to be formatted according to the original RFC 822 specification.

  • The Content-Type: header line indicates the type of data that is carried inside the message (see below)

  • The Content-Transfer-Encoding: header line is used to specify how the message has been encoded. When MIME was designed, some email servers were only able to process messages containing characters encoded using the 7 bits ASCII character set. MIME allows the utilization of other character encodings.

Inside the email header, the Content-Type: header line indicates how the MIME email message is structured. RFC 2046 defines the utilization of this header line. The two most common structures for MIME messages are :

  • Content-Type: multipart/mixed. This header line indicates that the MIME message contains several independent parts. For example, such a message may contain a part in plain text and a binary file.

  • Content-Type: multipart/alternative. This header line indicates that the MIME message contains several representations of the same information. For example, a multipart/alternative message may contain both a plain text and an HTML version of the same text.

To support these two types of MIME messages, the recipient of a message must be able to extract the different parts from the message. In RFC 822, an empty line was used to separate the header lines from the body. Using an empty line to separate the different parts of an email body would be difficult as the body of email messages often contains one or more empty lines. Another possible option would be to define a special line, e.g. *-LAST_LINE-* to mark the boundary between two parts of a MIME message. Unfortunately, this is not possible as some emails may contain this string in their body (e.g. emails sent to students to explain the format of MIME messages). To solve this problem, the Content-Type: header line contains a second parameter that specifies the string that has been used by the sender of the MIME message to delineate the different parts. In practice, this string is often chosen randomly by the mail client.

The email message below, copied from RFC 2046 shows a MIME message containing two parts that are both in plain text and encoded using the ASCII character set. The string simple boundary is defined in the Content-Type: header as the marker for the boundary between two successive parts. Another example of MIME messages may be found in RFC 2046.

Date: Mon, 20 Sep 1999 16:33:16 +0200
From: Nathaniel Borenstein <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
Subject: Test
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="simple boundary"

preamble, to be ignored

--simple boundary
Content-Type: text/plain; charset=us-ascii

First part

--simple boundary
Content-Type: text/plain; charset=us-ascii

Second part
--simple boundary

The Content-Type: header can also be used inside a MIME part. In this case, it indicates the type of data placed in this part. Each data type is specified as a type followed by a subtype. A detailed description may be found in RFC 2046. Some of the most popular Content-Type: header lines are :

  • text. The message part contains information in textual format. There are several subtypes : text/plain for regular ASCII text, text/html defined in RFC 2854 for documents in HTML format or the text/enriched format defined in RFC 1896. The Content-Type: header line may contain a second parameter that specifies the character set used to encode the text. charset=us-ascii is the standard ASCII character table. Other frequent character sets include charset=UTF8 or charset=iso-8859-1. The list of standard character sets is maintained by IANA

  • image. The message part contains a binary representation of an image. The subtype indicates the format of the image such as gif, jpg or png.

  • audio. The message part contains an audio clip. The subtype indicates the format of the audio clip like wav or mp3

  • video. The message part contains a video clip. The subtype indicates the format of the video clip like avi or mp4

  • application. The message part contains binary information that was produced by the particular application listed as the subtype. Email clients use the subtype to launch the application that is able to decode the received binary information.

Note

From ASCII to Unicode

The first computers used different techniques to represent characters in memory and on disk. During the 1960s, computers began to exchange information via tape or telephone lines. Unfortunately, each vendor had its own proprietary character set and exchanging data between computers from different vendors was often difficult. The 7 bits ASCII character table RFC 20 set was adopted by several vendors and by many Internet protocols. However, ASCII became a problem with the internationalization of the Internet and the desire of more and more users to use character sets that support their own written language. A first attempt at solving this problem was the definition of the ISO-8859 character sets by ISO. This family of standards specified various character sets that allowed the representation of many European written languages by using 8 bits characters. Unfortunately, an 8-bits character set is not sufficient to support some widely used languages, such as those used in Asian countries. Fortunately, at the end of the 1980s, several computer scientists proposed to develop a standard that supports all written languages used on Earth today. The Unicode standard [Unicode] has now been adopted by most computer and software vendors. For example, Java always uses Unicode to manipulate characters, Python can handle both ASCII and Unicode characters. Internet applications are slowly moving towards complete support for the Unicode character sets, but moving from ASCII to Unicode is an important change that can have a huge impact on current deployed implementations. See for example, the work to completely internationalize email RFC 4952 and domain names RFC 5890.

The last MIME header line is Content-Transfer-Encoding:. This header line is used after the Content-Type: header line, within a message part, and specifies how the message part has been encoded. The default encoding is to use 7 bits ASCII. The most frequent encodings are quoted-printable and Base64. Both support encoding a sequence of bytes into a set of ASCII lines that can be safely transmitted by email servers. quoted-printable is defined in RFC 2045. We briefly describe base64 which is defined in RFC 2045 and RFC 4648.

Base64 divides the sequence of bytes to be encoded into groups of three bytes (with the last group possibly being partially filled). Each group of three bytes is then divided into four six-bit fields and each six bit field is encoded as a character from the table below.

Value

Encoding

Value

Encoding

Value

Encoding

Value

Encoding

0

A

17

R

34

i

51

z

1

B

18

S

35

j

52

0

2

C

19

T

36

k

53

1

3

D

20

U

37

l

54

2

4

E

21

V

38

m

55

3

5

F

22

W

39

n

56

4

6

G

23

X

40

o

57

5

7

H

24

Y

41

p

58

6

8

I

25

Z

42

q

59

7

9

J

26

a

43

r

60

8

10

K

27

b

44

s

61

9

11

L

28

c

45

t

62

+

12

M

29

d

46

u

63

/

13

N

30

e

47

v

14

O

31

f

48

w

15

P

32

g

49

x

16

Q

33

h

50

y

The example below, from RFC 4648, illustrates the Base64 encoding.

Input data

0x14fb9c03d97e

8-bit

00010100 11111011 10011100 00000011 11011001 01111110

6-bit

000101 001111 101110 011100 000000 111101 100101 111110

Decimal

5 15 46 28 0 61 37 62

Encoding

F P u c A 9 l +

The last point to be discussed about base64 is what happens when the length of the sequence of bytes to be encoded is not a multiple of three. In this case, the last group of bytes may contain one or two bytes instead of three. Base64 reserves the = character as a padding character. This character is used once when the last group contains two bytes and twice when it contains one byte as illustrated by the two examples below.

Input data

0x14

8-bit

00010100

6-bit

000101 000000

Decimal

5 0

Encoding

F A = =

Input data

0x14b9

8-bit

00010100 11111011

6-bit

000101 001111 101100

Decimal

5 15 44

Encoding

F P s =

Now that we have explained the format of the email messages, we can discuss how these messages can be exchanged through the Internet. The figure below illustrates the protocols that are used when Alice sends an email message to Bob. Alice prepares her email with an email client or on a webmail interface. To send her email to Bob, Alice’s client will use the Simple Mail Transfer Protocol (SMTP) to deliver her message to her SMTP server. Alice’s email client is configured with the name of the default SMTP server for her domain. There is usually at least one SMTP server per domain. To deliver the message, Alice’s SMTP server must find the SMTP server that contains Bob’s mailbox. This can be done by using the Mail eXchange (MX) records of the DNS. A set of MX records can be associated to each domain. Each MX record contains a numerical preference and the fully qualified domain name of a SMTP server that is able to deliver email messages destined to all valid email addresses of this domain. The DNS can return several MX records for a given domain. In this case, the server with the lowest numerical preference is used first RFC 2821. If this server is not reachable, the second most preferred server is used etc. Bob’s SMTP server will store the message sent by Alice until Bob retrieves it using a webmail interface or protocols such as the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP).

../_images/email-delivery.png

Email delivery protocols

The Simple Mail Transfer Protocol

The Simple Mail Transfer Protocol (SMTP) defined in RFC 5321 is a client-server protocol. The SMTP specification distinguishes between five types of processes involved in the delivery of email messages. Email messages are composed on a Mail User Agent (MUA). The MUA is usually either an email client or a webmail. The MUA sends the email message to a Mail Submission Agent (MSA). The MSA processes the received email and forwards it to the Mail Transmission Agent (MTA). The MTA is responsible for the transmission of the email, directly or via intermediate MTAs to the MTA of the destination domain. This destination MTA will then forward the message to the Mail Delivery Agent (MDA) where it will be accessed by the recipient’s MUA. SMTP is used for the interactions between MUA and MSA 3, MSA-MTA and MTA-MTA.

SMTP is a text-based protocol like many other application-layer protocols on the Internet. It relies on the byte-stream service. Servers listen on port 25. Clients send commands that are each composed of one line of ASCII text terminated by CR+LF. Servers reply by sending ASCII lines that contain a three digit numerical error/success code and optional comments.

The SMTP protocol, like most text-based protocols, is specified as a BNF. The full BNF is defined in RFC 5321. The main SMTP commands are defined by the BNF rules shown in the figure below.

../_images/smtp-bnf.png

BNF specification of the SMTP commands

In this BNF, atext corresponds to printable ASCII characters. This BNF rule is defined in RFC 5322. The five main commands are EHLO, MAIL FROM:, RCPT TO:, DATA and QUIT 4. Postmaster is the alias of the system administrator who is responsible for a given domain or SMTP server. All domains must have a Postmaster alias.

The SMTP responses are defined by the BNF shown in the figure below.

../_images/smtp-response.png

BNF specification of the SMTP responses

SMTP servers use structured reply codes containing three digits and an optional comment. The first digit of the reply code indicates whether the command was successful or not. A reply code of 2xy indicates that the command has been accepted. A reply code of 3xy indicates that the command has been accepted, but additional information from the client is expected. A reply code of 4xy indicates a transient negative reply. This means that for some reason, which is indicated by either the other digits or the comment, the command cannot be processed immediately, but there is some hope that the problem will only be transient. This is basically telling the client to try the same command again later. In contrast, a reply code of 5xy indicates a permanent failure or error. In this case, it is useless for the client to retry the same command later. Other application layer protocols such as FTP RFC 959 or HTTP RFC 2616 use a similar structure for their reply codes. Additional details about the other reply codes may be found in RFC 5321.

Examples of SMTP reply codes include the following :

500  Syntax error, command unrecognized
501  Syntax error in parameters or arguments
502  Command not implemented
503  Bad sequence of commands
220  <domain> Service ready
221  <domain> Service closing transmission channel
421  <domain> Service not available, closing transmission channel
250  Requested mail action okay, completed
450  Requested mail action not taken: mailbox unavailable
452  Requested action not taken: insufficient system storage
550  Requested action not taken: mailbox unavailable
354  Start mail input; end with <CRLF>.<CRLF>

The first four reply codes correspond to errors in the commands sent by the client. The fourth reply code would be sent by the server when the client sends commands in an incorrect order (e.g. the client tries to send an email before providing the destination address of the message). Reply code 220 is used by the server as the first message when it agrees to interact with the client. Reply code 221 is sent by the server before closing the underlying transport connection. Reply code 421 is returned when there is a problem (e.g. lack of memory/disk resources) that prevents the server from accepting the transport connection. Reply code 250 is the standard positive reply that indicates the success of the previous command. Reply codes 450 and 452 indicate that the destination mailbox is temporarily unavailable, for various reasons, while reply code 550 indicates that the mailbox does not exist or cannot be used for policy reasons. Reply code 354 indicates that the client can start transmitting its email message.

The transfer of an email message is performed in three phases. During the first phase, the client opens a transport connection with the server. Once the connection has been established, the client and the server exchange greetings messages (EHLO command). Most servers insist on receiving valid greeting messages and some of them drop the underlying transport connection if they do not receive a valid greeting. Once the greetings have been exchanged, the email transfer phase can start. During this phase, the client transfers one or more email messages by indicating the email address of the sender (MAIL FROM: command), the email address of the recipient (RCPT TO: command) followed by the headers and the body of the email message (DATA command). Once the client has finished sending all its queued email messages to the SMTP server, it terminates the SMTP association (QUIT command).

A successful transfer of an email message is shown below

S: 220 smtp.example.com ESMTP MTA information
C: EHLO mta.example.org
S: 250 Hello mta.example.org, glad to meet you
C: MAIL FROM:<alice@example.org>
S: 250 Ok
C: RCPT TO:<bob@example.com>
S: 250 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: From: "Alice Doe" <alice@example.org>
C: To: Bob Smith <bob@example.com>
C: Date: Mon, 9 Mar 2010 18:22:32 +0100
C: Subject: Hello
C:
C: Hello Bob
C: This is a small message containing 4 lines of text.
C: Best regards,
C: Alice
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye

In the example above, the MTA running on mta.example.org opens a TCP connection to the SMTP server on host smtp.example.com. The lines prefixed with S: (resp. C:) are the responses sent by the server (resp. the commands sent by the client). The server sends its greetings as soon as the TCP connection has been established. The client then sends the EHLO command with its fully qualified domain name. The server replies with reply-code 250 and sends its greetings. The SMTP association can now be used to exchange an email.

To send an email, the client must first provide the address of the recipient with RCPT TO:. Then it uses the MAIL FROM: with the address of the sender. Both the recipient and the sender are accepted by the server. The client can now issue the DATA command to start the transfer of the email message. After having received the 354 reply code, the client sends the headers and the body of its email message. The client indicates the end of the message by sending a line containing only the . (dot) character 5. The server confirms that the email message has been queued for delivery or transmission with a reply code of 250. The client issues the QUIT command to close the session and the server confirms with reply-code 221, before closing the TCP connection.

Note

Open SMTP relays and spam

Since its creation in 1971, email has been a very useful tool that is used by many users to exchange lots of information. In the early days, all SMTP servers were open and anyone could use them to forward emails towards their final destination. Unfortunately, over the years, some unscrupulous users have found ways to use email for marketing purposes or to send malware. The first documented abuse of email for marketing purposes occurred in 1978 when a marketer who worked for a computer vendor sent a marketing email to many ARPANET users. At that time, the ARPANET could only be used for research purposes and this was an abuse of the acceptable use policy. Unfortunately, given the extremely low cost of sending emails, the problem of unsolicited emails has not stopped. Unsolicited emails are now called spam and a study carried out by ENISA in 2009 reveals that 95% of email was spam and this number seems to continue to grow. This places a burden on the email infrastructure of Internet Service Providers and large companies that need to process many useless messages.

Given the amount of spam messages, SMTP servers are no longer open RFC 5068. Several extensions to SMTP have been developed in recent years to deal with this problem. For example, the SMTP authentication scheme defined in RFC 4954 can be used by an SMTP server to authenticate a client. Several techniques have also been proposed to allow SMTP servers to authenticate the messages sent by their users RFC 4870 RFC 4871 .

The Post Office Protocol

When the first versions of SMTP were designed, the Internet was composed of minicomputers that were used by an entire university department or research lab. These minicomputers were used by many users at the same time. Email was mainly used to send messages from a user on a given host to another user on a remote host. At that time, SMTP was the only protocol involved in the delivery of the emails as all hosts attached to the network were running an SMTP server. On such hosts, an email destined to local users was delivered by placing the email in a special directory or file owned by the user. However, the introduction of personal computers in the 1980s, changed this environment. Initially, users of these personal computers used applications such as telnet to open a remote session on the local minicomputer to read their email. This was not user-friendly. A better solution appeared with the development of user friendly email client applications on personal computers. Several protocols were designed to allow these client applications to retrieve the email messages destined to a user from his/her server. Two of these protocols became popular and are still used today. The Post Office Protocol (POP), defined in RFC 1939, is the simplest one. It allows a client to download all the messages destined to a given user from his/her email server. We describe POP briefly in this section. The second protocol is the Internet Message Access Protocol (IMAP), defined in RFC 3501. IMAP is more powerful, but also more complex than POP. IMAP was designed to allow client applications to efficiently access in real-time to messages stored in various folders on servers. IMAP assumes that all the messages of a given user are stored on a server and provides the functions that are necessary to search, download, delete or filter messages.

POP is another example of a simple line-based protocol. POP runs above the bytestream service. A POP server usually listens to port 110. A POP session is composed of three parts : an authorisation phase during which the server verifies the client’s credential, a transaction phase during which the client downloads messages and an update phase that concludes the session. The client sends commands and the server replies are prefixed by +OK to indicate a successful command or by -ERR to indicate errors.

When a client opens a transport connection with the POP server, the latter sends as banner an ASCII-line starting with +OK. The POP session is at that time in the authorisation phase. In this phase, the client can send its username (resp. password) with the USER (resp. PASS) command. The server replies with +OK if the username (resp. password) is valid and -ERR otherwise.

Once the username and password have been validated, the POP session enters in the transaction phase. In this phase, the client can issue several commands. The STAT command is used to retrieve the status of the server. Upon reception of this command, the server replies with a line that contains +OK followed by the number of messages in the mailbox and the total size of the mailbox in bytes. The RETR command, followed by a space and an integer, is used to retrieve the nth message of the mailbox. The DELE command is used to mark for deletion the nth message of the mailbox.

Once the client has retrieved and possibly deleted the emails contained in the mailbox, it must issue the QUIT command. This command terminates the POP session and allows the server to delete all the messages that have been marked for deletion by using the DELE command.

The figure below provides a simple POP session. All lines prefixed with C: (resp. S:) are sent by the client (resp. server).

S:    +OK POP3 server ready
C:    USER alice
S:    +OK
C     PASS 12345pass
S:    +OK alice's maildrop has 2 messages (620 octets)
C:    STAT
S:    +OK 2 620
C:    LIST
S:    +OK 2 messages (620 octets)
S:    1 120
S:    2 500
S:    .
C:    RETR 1
S:    +OK 120 octets
S:    <the POP3 server sends message 1>
S:    .
C:    DELE 1
S:    +OK message 1 deleted
C:    QUIT
S:    +OK POP3 server signing off (1 message left)

In this example, a POP client contacts a POP server on behalf of the user named alice. Note that in this example, Alice’s password is sent in clear by the client. This implies that if someone is able to capture the packets sent by Alice, he will know Alice’s password 6. Then Alice’s client issues the STAT command to know the number of messages that are stored in her mailbox. It then retrieves and deletes the first message of the mailbox.

Footnotes

1

The list of all standard email header lines may be found at http://www.iana.org/assignments/message-headers/message-header-index.html

2

It could be surprising that the To: is not mandatory inside an email message. While most email messages will contain this header line an email that does not contain a To: header line and that relies on the bcc: to specify the recipient is valid as well.

3

During the last years, many Internet Service Providers, campus and enterprise networks have deployed SMTP extensions RFC 4954 on their MSAs. These extensions force the MUAs to be authenticated before the MSA accepts an email message from the MUA.

4

The first versions of SMTP used HELO as the first command sent by a client to a SMTP server. When SMTP was extended to support newer features such as 8 bits characters, it was necessary to allow a server to recognize whether it was interacting with a client that supported the extensions or not. EHLO became mandatory with the publication of RFC 2821.

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This implies that a valid email message cannot contain a line with one dot followed by CR and LF. If a user types such a line in an email, his email client will automatically add a space character before or after the dot when sending the message over SMTP.

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RFC 1939 defines the APOP authentication scheme that is not vulnerable to such attacks.