Simple Mail Transfer Protocol ( SMTP)
SMTP is the principal application layer protocol for internet electronic mail. It uses the reliable data transfer service of TCP to transfer mail from the sender mail from the sender small server the recipient mail server. SMTP has two sides a client side which executes on the sender mail server and a server side which executes on the recipient mail server both the client and server sides of SMTP run on every mail server when a mail server sends mail to other mail servers it acts as an SMTP client. When a mail server receive mail from other mail servers it acts as an SMTP server.
To illustrate the basic operation of SMTP let go through a common scenario suppose heena wants to send Harish a simple ASCII message to Harish.
1.Heena invokes her user agent for e mail provides Harish e mail address compose a message and instructs the user agent to send to message.
2.Heena user agent sends the message to her mail server wher it is placed in message queue.
3.The client side of SMTP running on heena mail server see the message in the message queue. It opens TCP connection to an SMTP server running on Harish mail server.
4. After some initial SMTP handshaking the SMTP receives the message Harish mail server then places the message in Harish mail box.
5.Harish invokes his user agent to read the message at his convenience.