Message transfer agent : SMTP :

• The actual mail transfer is carried out through the message transfer agent. A system should have the client MTA in order to send a mail and it should have a server MTA in order to receive one.

• SMTP is the protocol which defines MTA client and server in the internet.

• As shown in fig. the SMPT is used twice, once between the sender and sender's mail server and then between the two mail servers.

• The job of SMTP is simply to define how commands and responses be sent back and forth. Each network can choose its software package for implementation.

Commands and Responses :

• As shown in fig SMTP the transfer of messages between MTA client and MTA server server takes place using the command and response principle.

• Each command or response is terminated by a two character end of line token. the two characters used are carriage return and life feed.

SMTP (Simple mail transfer protocol) operation :

• In internet the source machine establishes a connection to port 25 of the destination machine so as to deliver an email.

• An e-mail daemon which speaks SMTP is listening to this port

• This demon is supposed to perform the following tasks :

  1. Accept the incoming connections and copy messages from them into appropriate mailboxes.

  2. Return an error message to the sender, if a message is not delivered.

• SMTP is a simple ASCII protocol

• Once a TCP connection between a sender and port 25 of the receiver is established, the sending machine operates as a client and the receiving machine acts as a server.

• The client then waits for the server to take initiative in communication

• The server sends a line of text which declares its identity and announces its willingness/ unwillingness to receive mail. if the server is not prepared, the client will release the connection, wait for sometime and try again later

• But if the server is willing to accept e-mail, then the client announces the sender of e-mail and its recipient.

• If such a recipient exists at the destination, then the server tells the client to send a message. the client then sends the message and the server sends back its acknowledgement.

• No checksums are generally required because TCP provides a reliable byte stream. if there are any more email, then they can be sent now

• After exchanging all the email, the connection is released

• SMTP uses numerical codes, the lines sent by the client are marked C : : and those sent by the server are marked S : :

• Some of the commands, useful for communication are :

  HELO, RCTP, DATA, QUIT etc.

• RCTP represents recipient. if only one command is used then the message is being sent to only one recipient. if the command is used many times, then it indicates that the message is sent to more than one recipients.

• In such a case each message is individually acknowledged or rejected

• The syntax of four character commands for the client are rigidly specified but the syntax for the replies are not that rigid

• The SMTP protocol is well defined by RFC 821 but some problems are still present.

Problems in SMTP :

Some of the problems in SMTP are as follows:

1. Some older versions of SMTP are not capable of handling messages longer than 64KB

2. If client and server have different time outs, then one of them may give up when the other is still busy. this will terminate the connection unnecessarily.

3. In rate situations, infinite mailstorms can be triggered.

Extended SMTP (ESMTP) :

Some of these problems can be resolved by using the extended SMTP (ESMTP) which is defined in RFC 1425

Message Access Agent: POP and IMAP :

• The SMTP is used in the first and second stages of mail delivery. But SMTP is not used in the third stage, because SMTP is a push protocol which is meant for pushing the message from client to server

• The third stage needs a pull protocol because the client has to pull messages from the server. the bulk data gets transffered from the server to the client. therefore third stage uses a message access agent which is a pull protocol

• The two message access agents avaible are :

1. Post office protocol, version 2 (POP 3)

2. Internet mail access protocol (IMAP 4)

POP 3:

• The POP 3 consists of client POP3 software and server POP 3 sftware. out of these, the client POP3 software is installed on the receiving computer whereas the mail server gets the server POP 3 software installed on it.

• Whenthe user wants to download email from the mailbox on the email server, the events take place in the following sequence.

1. The client (user) establishes a connection with the server on TCP port 110

2. The client then sends its user name and password to the server in order to access the mailbox

3. The user is then allowed to list and get the messages one by one

• This is called as downloading.

Modes of POP 3:

POP3 has two modes of operation :

1. Delete mode and

2. Keep mode.

Delete mode : In this mode the mail is deleted from the mailbox after each retrieval. This mode is used when the user is working on his permanent computer because it is then possible for him to save and rearrange the received mail after reading it.

Keep mode :

If operated in this mode, the mail remains in the mailbox after retrival. this mode is used when the user accesses mail away from the primary computer. then read mail can be organized later.

Disadvantages of POP 3:

1. POP 3 does not allow organisation of email on the server

2. The user cannot create different folders on the server. it can create them only on his own computer

3. The user cannot partially check the contents of E mail before downloading

IMAP 4 :

• Internet mail access protocol version 4 (IMAP 4) is another mail access protocol which is very similar to POP 3 but has more features

• This makes IMAP 4 more powerful but more complex as compared to POP3

• IMAP is more sophisticated than POP 3 and it is defined in RFC 1064.