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F P s \=
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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 (:term:`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 (:term:`POP`) or the Internet Message Access Protocol (:term:`IMAP`).
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The Simple Mail Transfer Protocol
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The Simple Mail Transfer Protocol (:term:`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 [#fsmtpauth]_, MSA-MTA and MTA-MTA.
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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.
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The SMTP protocol, like most text-based protocols, is specified as a :term:`BNF`. The full BNF is defined in :rfc:`5321`. The main SMTP commands are defined by the BNF rules shown in the figure below.
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BNF specification of the SMTP commands
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In this BNF, `atext` corresponds to printable ASCII characters. This BNF rule is defined in :rfc:`5322`. The five main commands are `EHLO` [#fehlo]_, `MAIL FROM:`, `RCPT TO:`, `DATA` and `QUIT`. `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.
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The SMTP responses are defined by the BNF shown in the figure below.
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BNF specification of the SMTP responses
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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`.
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Examples of SMTP reply codes include the following :
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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 `250` is the standard positive reply that indicates the success of the previous command. Reply code `354` indicates that the client can start transmitting its email message. 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 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. The `500` to `503` reply codes correspond to errors in the commands sent by the client. The `503` 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).
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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).
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A successful transfer of an email message is shown below
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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.
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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 [#fdot]_. 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.
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Open SMTP relays and spam
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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 <http://www.templetons.com/brad/spamreact.html#msg>`_ 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 <http://www.enisa.europa.eu/act/res/other-areas/anti-spam-measures>`_ 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.
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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` .
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