2 , or electronic mail, is a method of exchanging digital messages from an author to one or more recipients.
3 Early systems required the author and recipient to both be online at the same time.Similar to instant messaging
4 Email Today’s email systems are based on a store- and-forward model. servers accept, forward, deliver, and store messages.Neither the users nor their computers are required to be online simultaneously.
12 SMTPSMTP 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
13 SMTPAn 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)
14 SMTPMAIL- 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.
16 POPPost Office Protocol (POP) is an application- layer Internet standard protocol used by local clients to retrieve from a remote server over a TCP/IP connection.POP has been developed through several versions, with version 3 (POP3) being the current standard
17 POP Virtually all modern email clients and servers support POP3. POP3 along with IMAP are the two most prevalent Internet standard protocols for retrieval.
18 POPSupports simple download-and-delete requirements for access to remote mailboxesAlthough most POP clients have an option to leave mail on server after download, clients generally connect, retrieve all messages, store them on PC, delete from server, and then disconnect
19 POPThe 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 foldersPOP3 is the interface between clients and the “mail store”
21 POPThe 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.
22 POPThe 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.
23 POP 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
24 POPThe SMTP delivery service for POP3 is the component that transfers from the SMTP service to the user mailboxes.The delivery service is notified by the SMTP service when new arrives.New is delivered to the mail store by means of the Mail Storage Access API
25 POPThe 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.
27 IMAPIMAP (Internet Message Access Protocol) is an application layer internet protocol that allows an client to access on a remote server.Again, IMAP along with POP3 make up the two most prevalent Internet standard protocols for retrieval.
28 IMAP Supports both online and offline modes of operation. 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.
29 IMAPIncoming messages are sent to an server that stores messages in the recipients box.The user retrieves the messages with an client that uses on of a number of retrieval protocols.
31 IMAP Advantages over POP: When using POP, clients typically connect to the server briefly, only as long as it takes to download new messages.With IMAP, clients often stay connected as long as the user interface is active and download message content on demand.For users with many or large messages, this can result in faster response times.
32 IMAP AdvantagesUnlike 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.
33 IMAP AdvantagesMost internet 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.
34 IMAP AdvantagesThrough 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.
35 IMAP Advantages 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.
36 IMAPWhile IMAP remedies many shortcomings of POP, this introduces additional complexity.Some problems can arise from this, such as tying up server resources.
37 MIMEMultipurpose Internet Mail Extensions (MIME) is an internet standard that extends the format of to support:Text in character set other than ASCIINon-text attachmentsMessage bodies with multiple partsHeader information in non-ASCII character sets
38 MIME Originally designed for SMTP protocol. Has grown beyond describing the content of 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 is transmitted via SMTP in MIME format.
39 MIMEMime uses headers as a way to describe the structure of the .Common MIME headers:MIME-VersionContent-TypeContent-Transfer-EncodingContent-IDContent-DescriptionContent-Disposition
40 MIME 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.
41 MIME 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.
42 MIMEContent-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.
43 MIMEContent-Description: optional header used to add descriptive text to non-textual body partsContent-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.