SMTP (Simple Mail Transfer Protocol)? Brief discussion of SMTP

SMTP:

When we send a mail it travels between many different computers on the way from sender to receiver. The first part is SMTP server and we must have to communicate with this server using SMTP protocol.

SMTP stands for Simple Mail Transfer Protocol is the standard technique used by people around the globe while sending mail. 

Client communicate with SMTP server using SMTP protocol. It is simply a text based protocol which uses some command & reply codes for its communication. The main objective of SMTP is to send mail message reliably, efficiently and easily.

SMTP uses port 25 

Normally, SMTP communicates with the outside world through TCP port 25. Client who wants to communicate with SMTP server must have to establish a connection through TCP port 25. However some mail server uses different port to communicate with SMTP server for example Port 2525.

SMTP commands

Below are the basic commands used by the SMTP server and  client to communicate with each other: HELO, MAIL FROM, RCPT TO, DATA, RSET, QUIT, HELP, VRFY etc. 

The actual  mail transfer is done by MTA (Message Transfer Agent). To send mail, system must have MTA client & to receive a mail, system must have MTA server.

Process of transferring mail occurs in 3 phases: 

• Connection Establishment 
•  Mail transfer 
• Connection Termination

NB: 

Code 220 means  Service ready, 

421 Service not ready, 

250 Request command completed, 

354 start mail input and  221 service closed.

 (As per given diagram you can explain more in your own language)