SMTP | Simple Mail Transfer Protocol | Application Layer

SMTP used as the common mechanism for transporting electronic mail among different host within TCP/IP.

SMTP is an application layer protocol under SMTP,a client SMTP process opens a TCP connection to a server SMTP process on a remote host and attempts to send mail across the connection.

The server SMTP listens for a TCP connection on a specific port and the client SMTP process initiates a connection on that port.

When the TCP connection is successful the two process execute a simple request-response dialogue,in which the client process transmits the mail addresses of the originator and the recipient for a message.

When the server process accepts these mail addresses,the client process transmits the email instant message.The message must contain message header and message text. 

SMTP MODEL AND PROTOCOL: 

SMTP model supports both end-to-end message transfer and store and forward mail delivery methods.

The TFC 821 standard defines 9 client-server protocol.The client SM initiated the session and server responds to the session request.

An SMTP based process can transfer electronic mail to another process on the same network or to another network via a relay or gateway. 

An email message may pass through a number of intermediate relay or gateway.

The exchange of mail using TCP is performed by MTA.MTA maintains a mail queue so that it can schedule repeat delivery attempts.

Local MTA delivers mail to mailboxes and the information can be downloaded by UA[User Agent].SMTP client and server should have two components.They are UA and Local MTA. 

The senders UA prepares the message,creates the envelope and puts message in the envelope.The MTA transfers the mail across the network to the TCP Port 25 of the receivers MTA. 

USER AGENT: 

SENDING E-MAIL:

E-mail is sent by series of request-response transactions between client and server.An SMTP transactions consists of the envelope and message which is composed of header.The message body contains the actual information that can be read by the recipient. 

MAIL HEADER:

The header includes sending date,sender's address and receivers address.Some frequently used fields are to,cc,from,subject,bcc etc. 

RECEIVING EMAIL:

The UA periodically checks the content of mail boxes.It informs the user about mail arrival by giving a special notice. 

MAIL TRANSFER AGENT:

MTA's transfer actual mail.The system must have the client MTA for sending email and the server MTA for receiving email. 

EXAMPLE: 

HELLO cs.princeton.edu 

250 hello daemon@mail.cs.princeton.edu[128.12.169.24] 

MAIL FROM:

250 OK 

RCPT TO:

250 OK 

RCPT TO:

550 No such user here. 

DATA 

354 start mail input;end with>.

hi hi hi... . 

RLF>.

 250 OK 

QUIT 

221 closing connection.