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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 <http://en.wikipedia.org/wiki/Comparison_of_mail_servers>`_. More detailed information about the full Internet Mail Architecture may be found in :rfc:`5598`.
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Electronic mail
Messagerie électronique
Electronic mail, or email, is a very popular application in computer networks such as the Internet. Email `appeared <http://openmap.bbn.com/~tomlinso/ray/firstemailframe.html>`_ 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.
Les applications de messagerie électronique, ou email, sont populaires sur les réseaux informatiques comme Internet. L'email `est apparu <http://openmap.bbn.com/~tomlinso/ray/firstemailframe.html>`_ au début des années 1970 et permet aux utilisateurs d'échanger des messages sous forme textuelle. A la base, il était utilisé pour échanger de courts messages, mais sont utilisation n'a fait qu'augmenter ces dernières années. Il n'est plus seulement utilisé pour échanger de courts messages, mais également de longs messages pouvant être composés de différentes parties telles que nous le verrons par après.
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 <http://en.wikipedia.org/wiki/Comparison_of_mail_servers>`_ 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.
 
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 <http://en.wikipedia.org/wiki/Comparison_of_mail_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.
 
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 [#fto]_. 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.
 
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 [#femailheaders]_. 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.
 

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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 <http://en.wikipedia.org/wiki/Comparison_of_mail_servers>`_. More detailed information about the full Internet Mail Architecture may be found in :rfc:`5598`.
4 years ago
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Source string location
../../protocols/email.rst:54
String age
4 years ago
Last updated
2 months ago
Source string age
4 years ago
Translation file
locale/fr/LC_MESSAGES/protocols/email.po, string 9