E-MAIL

E-mail is a most familiar and widely used network service. It is a system for sending messages or files to other computer users based on mailbox addresses rather than a direct host-to host exchange, and supports mail exchange between users on the same or different computers. Unlike other client-server applications, E-mail allows users to send anything from short notes to extensive files without worrying about the current availability of the receiving host. Some e-mail uses are as follows:

• Send a single message to one or many recipients.
• Send messages that include text, voice, video, or graphics.
• Send messages to users on networks outside the Internet.
• Send messages calling for a response from a computer program rather than a user.

E-mail design is similar to the postal system. Addresses are used to identify both the recipient and sender of a message  (return address). Messages that cannot be delivered within a specified amount of time are returned to the sender. Every user on the network has a private mailbox. Received mail is stored in the mailbox until the recipient removes or discards it.

Electronic mail is different from other message transfer services provided by the Internet in the sense that it provides a mechanism called spooling, which allows a user to send mail even if a network is currently disconnected or the receiving machine is not operational. When a message is sent, a copy is placed in a storage facility called a spool. A spool resembles a queue. Messages in a queue are processed on a first-come, first-served basis. Once in a spool, a message is searched every 30 seconds by a client process running in the background. The background client looks for new messages and not-yet-sent old messages and attempts delivery. If the client process is unable to deliver a message, it marks the message with the time of the attempted delivery, leaves it in the spool, and repeats the attempt at a later time. If all attempts at delivery fail, the message may be deemed undeliverable after several days and returned to the mailbox of the sender. A message is considered delivered only when both client and server agree that the recipient has seen and disposed of it. Until then copies are kept in both the sending spool and the receiving mailbox.

An E-mail system consists of two sub systems:

User Agent (UA): A user agent supports user interface by allowing people to read and send E-mail. It retrieves incoming messages, mails outgoing messages and helps to compose messages by providing command based, menu based or graphical method to interaction.

Message Transfer Agent (MTA): Move messages from the source to the destination. It serves as the mail system’s interface with the network and runs in the background to move E-mail through the system.

Architecture & Services

An E-mail system support five basic functions or services:

1. Composition: Process of creating messages and answers. The system provides assistance with addressing and the numerous header fields attached to each message. For example, when answering a message, the e-mail system can extract the originator’s address from the incoming e-mail and automatically insert it in to the proper place in the reply.
2. Transfer: This refers to moving messages from the originator to the recipient. This requires establishing a connection to the destination or some intermediate machine, outputting the message, and releasing the connection.

3. Reporting: The originator is informed about what happened to the message – delivered, rejected or lost.

4. Displaying: This makes the incoming message displayed to the user so that he can read it. Necessary conversions like formatting, conversion of digitized voice etc. is done if required.
5. Disposition: This is concerned with what the recipient does with the message after receiving it. A message can be saved or deleted after reading or may be forwarded to some other person   .

Advanced features

6. Forwarding: This is an advanced feature in which an e-mail may be automatically forwarded to other user in case a recipient is away for some period of time.

7. Mailbox creation: A user can create a mailbox to store incoming E-mail. Mailboxes can be created and destroyed. Contents of mailboxes can be inspected and messages can be inserted and deleted from the mailboxes.

8. Mailing list: A message can be sent to a group of persons together with a single command. When a message is sent to the mailing list, identical copies are delivered to everyone on the list.

9. Registered E-mail: Allows the originator to know that his message has arrived. Automatic notification of undelivered mail is also done.

Other advanced features are carbon copies, high-priority e-mail, secret (encrypted) e-mail, alternative recipients if the primary one is not available, and the ability for secretaries to handle their boss’s e-mail.

User Agent

A user agent is a user interface that routes messages between the uses and the local MTA and accepts commands for composing, receiving and replying to messages as well for manipulating mailboxes menu or icon-driven interfaces through mouse or 1-character commands through keyboard may be provided by a user agent.

Thus, the user agent interacts with the user and essentially defines what the user can do.

A user agent also manages a message store (MS), which is used to store messages.

Some of the user agent’s functions are listed below:

• Send Mail. The User Agent (UA) accepts a message and address from a user and gives it to the MTA for delivery. The UA will invoke an editor to allow the user to write a message or it will simply send the contents of a previously constructed file (which presumably contains the message). The address may be DNS address of the form - mail box @ location or may be of other forms like defining attributes (attribute = value pairs), or mailing lists. User Agent also appends a header to the user message containing the following information:

(a)  sender

1. recipients
2. cc recipients (those who get copies)
3. blind copy recipients (those who get copies secretly)
4. subject (short note describing its contents)
5. message ID
6. reply-by date
7. sensitivity (degree of confidentiality)
8. priority

This information is useful to the receiving UA for delivering the message to the appropriate users and for displaying a summary of all mail messages in a user’s mailbox.

• Display a list of mail messages. This is similar to a directory list in a computer account that lists all files. In this case, the UA provides a list of all messages for the current user. Each entry in the list typically contains the following items:

1. source of the message
2. subject field
3. message size
4. various indicators indicating whether the message has been read, deleted, answered, or forwarded to another user.
5. message ID
6. date received

• Display the contents of a message on the computer or workstation screen.

• Reply. The UA allows the user to respond to a currently displayed message by invoking an editor through which the user can construct a response. When the user is finished, the UA automatically sends the response to the source of the previously displayed message.

• Forward the current message to one or more specified recipients.
• Extract the message and store it in a file. This is convenient if you want to save a message, print it, or make it available to others without explicitly forwarding it to them.

• Delete unwanted messages. Messages are not always physically deleted but rather are marked for deletion. Then another command such as a purge deletes all files marked for deletion. This allows accidentally deleted messages to be recovered.

• Undelete a message. Removes the deletion mark mentioned previously.

Message transfer agent (MTA)

The MTA is software running on a dedicated workstation or computer and is part of the email system’s backbone. Each MTA communicates with one or more UA’s and other MTAs. Its basic function is to accept mail from a UA or another MTA, examine it, and route it. For example, when it receives mail from a UA it verifies the format of the mail. If it is not correct it informs the UA an error has occurred so the sender can be notified. If it is correct there are two possibilities. First, the recipient is reachable via another UA to which the MTA is connected. Example is user A sending mail to user B connected to same MTA. In this case the MTA gives the mail to the appropriate UA for delivery. Second, the UA that will deliver the mail is connected to another MTA. Here user A may have sent mail to user C connected to some other MTA. In this case the mail must be routed to another MTA. Collectively the MTAs execute a routing strategy that sends the mail through one or more MTAs until it reaches the desired one. Then the mail is sent to the appropriate UA for delivery.

The UA appends a header to a message sent from the user. If the MTA must route the mail to another MTA it also appends additional bytes to the message and header. These additional bytes are called the envelope. The envelope is used by the MTAs for routing, error checking, and verification. The following are some of the fields in the envelope:

• Destination address
• Sender address for possible acknowledgment or return
• Mail identification number
• Priority
• Deferred date. A user may specify that delivery must occur after a given date.
• Delivery date. A user may specify that delivery must occur before a given date.
• Field specifying whether a message should be returned if it cannot be delivered
• Bytes for error detection
• Encryption information, such as the location of a key
• Digital signatures. This field can provide authentication for the sender. This is useful if a sender later denies transmitting something, or at least “cannot recall” sending it.

MESSAGE FORMAT AND TRANSFER

Messages consist of a primitive envelope, some number of header fields, a blank line, and then the message body. Each header field consists of a single line of ASCII text containing the field name, a colon, and, for most fields, a value.

The principal header fields related to message transport are:

To: E-mail address(es)(DNS address) of primary recipient(s)

Cc: E-mail address(es) of secondary recipient(s)

Bcc: E-mail address(es) for blind carbon copies. Here Bcc: (Blind Carbon Copy) field is like the Cc: field, except that the line is deleted from all the copies sent to the primary and secondary recipients. This feature allows people to send copies to third parties without the primary and secondary recipients knowing this.

From: person or people who created the message.

Sender: E-mail address of the actual sender. This field may be omitted if it is the same as the From field. Else, a sender may be a different person(for example secretary) than the actual creator of the message(for example a business executive).

Received: Line added by each message transfer agent along the route. The line contains the agent’s identity, the date and time the message was received, and other information that can be used for finding bugs in the routing system.

Return-path: can be used to identify a path back to the sender. This information can be gathered from all the Received: headers.

The message transfer system is concerned with relaying messages from originator to the recipient. The simplest way to do this is to establish a transport connection from the source machine to the destination machine and then just transfer the message.

SIMPLE MAIL TRANSFER PROTOCOL (SMTP)

SMTP is the basic protocol for transmitting messages between computers in the Internet.

Certainly one of the most common uses of networks is electronic mail, the ability to send a message or file to a specific user at a local or remote site. Typically, you send a message by specifying the email address of the recipient. The usual address format is name@host-text-address. The message is buffered as the destination site and is accessible only by the intended user.

There are some similarities with file transfer protocols. For example, both use a client and server to negotiate transfer of data. However, email typically sends the file to a specified user, in whose account the message is buffered. Also, the email client and server work in the background. For example, if you get a file-using FTP, you wait until the file arrives before doing another task. If you send (or receive) a file using email you can do other tasks while the client and server perform the mail delivery in the background. In the case of receiving mail you need not even be logged on. You will be notified of new mail the next time you log in to the system.

The standard mail protocol in the TCP/IP suite is the Simple Mail Transfer Protocol (SMTP). It runs above TCP/IP and below any local mail service. Its primary responsibility is to make sure mail is transferred between different hosts. By contrast, the local service is responsible for distributing mail to specific recipients.

When a user sends mail, the local mail facility determines whether the address is local or requires a connection to a remote site. In the latter case, the local mail facility stores the mail (much as you would put a letter in a mailbox), where it waits for the client SMTP. When the client SMTP delivers the mail, it first calls TCP to establish a connection with the remote site. When the connection is made, the client and server SMTPs exchange packets and eventually deliver the mail. At the remote end the local mail facility gets the mail and delivers it to the intended recipient.

The packets are also called SMTP protocol data units (PDUs) or simply commands. When the TCP connection is made the server sends a 220 PDU indicating it is ready to receive mail. The number 220 serves to identify the type of packet. Afterward, the client and server exchange the identities of their respective sites. Next, the client sends a MAIL FROM PDU indicating there is mail and identifying the sender. If the server is willing to accept mail from that sender it respond with a 250 OK PDU.

The server then sends one or more RCPT TO PDUs specifying the intended recipients to determine whether the recipients are there before sending the mail. For each recipient, the server responds with a 250 OK PDU (recipient exists) or a 550 recipient not here PDU. After the recipients have all been identified, the client sends a DATA PDU indicating it will begin mail transmission. The server’s response is a 354 start mail PDU, which gives the OK to start sending and specifies a sequence the client should use to mark the mail’s end. In this case the sequence is . . The client sends the mail in fixed-size PDUs, placing this sequence at the mail’s end. When the server gets the last PDU, it acknowledges receipt of the mail with another 250 OK PDU. Finally, the client and server exchange PDUs indicating they are ceasing mail delivery and TCP releases the connection.