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.