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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.
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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
 
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.
 
0x14
 
00010100
 
000101 000000
 
5 0
 
F A \= \=
 
0x14b9
 
00010100 11111011
 
000101 001111 101100
 
5 15 44
 
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 (: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`).
 
The Simple Mail Transfer Protocol
 
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.
 
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 :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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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.
4 years ago
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31097 String information

Source string location
../../protocols/email.rst:286
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 179