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SMTP, POP, IMAP, MIME

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Email, or electronic mail, is a method of exchanging digital messages from an author to one or more recipients.

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Early email systems required the author and recipient to both be online at the same time. Similar to instant messaging

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Todays email systems are based on a store- and-forward model. Email servers accept, forward, deliver, and store messages. Neither the users nor their computers are required to be online simultaneously.

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An email consists of 3 components: 1. Envelope 2. Header 3. Body

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Envelope- The envelope of an email is similar to a paper envelope for mail. It conceals the contents of the mail. Encrypts email into a data packet

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Header- contains control information including things like, sender address, recipient address, timestamp, cc, bc, etc. Body Text body of email

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SMTP Simple Mail Transfer Protocol Internet standard for electronic mail

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Network-based email was initially exchanged on the ARPANET in extensions to the FTP. In the 1970s SMTP grew out of these standards.

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Email is submitted by a mail client(MUA, mail user agent) to a mail server(MSA, mail submission agent) using SMTP. The MSA delivers the mail to its mail transfer agent (MTA).

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SMTP is a connection-oriented, text-based protocol in which a mail sender communicates with a mail receiver by issuing command strings and supplying necessary data over a reliable ordered data stream channel. This channel is typically a Transmission Control Protocol (TCP) connection

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An SMTP session consists of commands originated by an SMTP client. A session may include zero or more SMTP transactions. An SMTP transaction consists of 3 command/reply sequences (MAIL, RCPT, DATA)

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MAIL- command to establish the return address, reverse-path, bounce address, mfrom, or envelope sender. RCPT- command to establish a recipient of the message. DATA- signals the beginning of the message text; content of message rather than its envelope. Includes header and body.

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Post Office Protocol (POP) is an application- layer Internet standard protocol used by local email clients to retrieve email from a remote server over a TCP/IP connection. POP has been developed through several versions, with version 3 (POP3) being the current standard

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Virtually all modern email clients and servers support POP3. POP3 along with IMAP are the two most prevalent Internet standard protocols for email retrieval.

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Supports simple download-and-delete requirements for access to remote mailboxes Although most POP clients have an option to leave mail on server after download, email clients generally connect, retrieve all messages, store them on PC, delete from server, and then disconnect

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The POP3 service uses the SMTP service to provide inbound message delivery as well as outbound message sending for POP3 clients. Treats mailbox as one store, no concept of folders POP3 is the interface between email clients and the mail store

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The authentication store is the repository of user information needed to authenticate the user. The authentication module accesses the authentication store to verify the credentials submitted by the client to the POP3 service.

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The Mail Storage API is the common interface to the mail store for all processes. The POP3 service, SMTP delivery service for POP3, and POP3 Server Administrator use the API to access the mail store.

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The mail store uses the file system for storage. Typically located on the same server as the POP3 service, but should be located on a different local or network volume to avoid disk space problems. Even though contained in file system, accessed by using Mail Storage Access API

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The SMTP delivery service for POP3 is the component that transfers email from the SMTP service to the user mailboxes. The delivery service is notified by the SMTP service when new email arrives. New email is delivered to the mail store by means of the Mail Storage Access API

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The POP3 Server Administrator is the interface used by the tools to configure the server, domains, and mailboxes. The server administrator accesses the mail store through API to create or modify domains and mailboxes. Modifications to the service itself are made through calls to the administration interface within the POP3 service.

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IMAP (Internet Message Access Protocol) is an application layer internet protocol that allows an email client to access email on a remote server. Again, IMAP along with POP3 make up the two most prevalent Internet standard protocols for email retrieval.

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Supports both online and offline modes of operation. Email clients using IMAP generally leave messages on the server until the user explicitly deletes them. This allows for multiple clients to manage the same mailbox.

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Incoming email messages are sent to an email server that stores messages in the recipients email box. The user retrieves the messages with an email client that uses on of a number of email retrieval protocols.

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Advantages over POP: 1. When using POP, clients typically connect to the email server briefly, only as long as it takes to download new messages. 2. With IMAP, clients often stay connected as long as the user interface is active and download message content on demand. 3. For users with many or large messages, this can result in faster response times.

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Unlike POP where the currently connected client can be the only client connected to the mailbox, IMAP allows for simultaneous access by multiple clients. Provides mechanisms for clients to detect changes made to the mailbox by other currently connected clients.

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Most internet email is transmitted in MIME format (discussed later) IMAP allows clients to retrieve any of the individual MIME parts separately and also to retrieve portions of either individual parts or the entire message. This allows clients to retrieve the text portion of a message without retrieving attached files or to stream content as it is being fetched.

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Through the use of flags, IMAP allows clients to keep track of message states. Whether or not the message has been read, replied to, or deleted. These flags are stored on server, so different clients accessing the same mailbox at different times can detect state changes.

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IMAP allows for multiple mailboxes on the server IMAP clients can create, rename, and/or delete mailboxes on the server, and copy messages between mailboxes. Provides a search mechanism for searching a server for messages meeting a variety of criteria.

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While IMAP remedies many shortcomings of POP, this introduces additional complexity. Some problems can arise from this, such as tying up server resources.

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Multipurpose Internet Mail Extensions (MIME) is an internet standard that extends the format of email to support: 1. Text in character set other than ASCII 2. Non-text attachments 3. Message bodies with multiple parts 4. Header information in non-ASCII character sets

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Originally designed for SMTP protocol. Has grown beyond describing the content of email and now often includes describe content type in general, including for the web, and as a storage for rich content in some commercial products. Virtually all email is transmitted via SMTP in MIME format.

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Mime uses headers as a way to describe the structure of the email. Common MIME headers: 1. MIME-Version 2. Content-Type 3. Content-Transfer-Encoding 4. Content-ID 5. Content-Description 6. Content-Disposition

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MIME-Version Ex: MIME-Version: 1.0 Required header indicating this message is to use the rules of MIME. Content-Type: used to specify the media type and subtype of data in the body of a message and to fully specify the native representation of such data.

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Content-Transfer-Encoding: can have 2 different meanings If the value is base64 or quoted-printable, then the header indicates the encoding used for this body part. If the value is 7bit, 8bit, or binary, then the header indicates that there is no encoding the that this value indicates only the type of content this body part contains.

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Content-ID: world-unique values that identify body parts, individually or as groups. Necessary at times to distinguish body parts and allow cross-referencing between body parts.

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Content-Description: optional header used to add descriptive text to non-textual body parts Content-Disposition: provides information about how to present a message or a body part. When a body part is to be treated as an attached file, the Content-Disposition header will include a file name parameter.

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(Questions?)