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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.
the `In-reply-to:` header line 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.
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 (:term:`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 line 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.

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../../protocols/email.rst:107
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locale/pot/protocols/email.pot, string 20