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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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