What is simple mail transfer protocol (SMTP) and how can work?

What is SMTP?

SMTP is part of the application layer of the TCP/IP protocol. Using a process called "store and forward," SMTP moves your email on and across networks. It works closely with something called the Mail Transfer Agent (MTA) to send your communication to the right computer and email inbox.

SMTP spells out and directs how your email moves from your computer's MTA to an MTA on another computer, and even several computers. Using that "store and forward" feature mentioned before, the message can move in steps from your computer to its destination. At each step, Simple Mail Transfer Protocol is doing its job. Lucky for us, this all takes place behind the scenes, and we don't need to understand or operate SMTP.

How Simple Mail Transfer Protocol (SMTP) works

Simple Mail Transfer Protocol (SMTP) is based on end-to-end message delivery. An Simple Mail Transfer Protocol (SMTP) client contacts the destination host's Simple Mail Transfer Protocol (SMTP) server on well-known port25, to deliver the mail. The client then waits for the server to send a 220 READY FOR MAIL message. Upon receipt of the 220 message, the client sends a HELO command. The server then responds with a "250 Requested mail action okay" message.

After this, the mail transaction will begin with a MAIL command that gives the sender identification as well as a FROM: field that contains the address to which errors should be reported.

After a successful MAIL command, the sender issues a series of RCPT commands that idenclfy recipients of the mail message. The receiver will the acknowledge each RCPT command by sending 250 OK or by sending the error message 550 No such user here.

After all RCPT commands have been acknowledged, the sender issues a DATA command to inform the receiver that the sender is ready to transfer a complete mail message. The receiver responds with message 354 Start mail command with an ending sequence that the sender should use to terminate the message data. The termination sequence consists of 5 characters: carriage return, line feed, period, carriage return, and line feed (<CRLF>.<CRLF>).

The client now sends the data line by line, ending with the 5-character sequence <CRLF>.<CRLF> line, upon which the receiver will acknowledge with a 250 OK, or an appropriate error message if anything went wrong.

After the sending is completed, the client can follow any of these actions.

Terminate Session: If the current Simple Mail Transfer Protocol (SMTP) client has no more messages to send, the connection can be closed with a QUIT command, which will be answered with a 221 Service closing transmission channel reply.

Exchange Roles: If the current Simple Mail Transfer Protoco (SMTP) client has no more messages to send, but is ready to receive any messages from the current Simple Mail Transfer Protoco (SMTP) server, it can issue the TURN command. Now the SMTP client and the SMTP server will switch their role of sender/receiver, and the sender (previous receiver) can now send messages by issuing a MAIL command.

Send Another Mail: If the Simple Mail Transfer Protoco (SMTP) client (sender) has another message to send, it can issue a new MAIL command.

This lesson explains Simple Mail Transfer Protocol (SMTP), the RFC's related with Simple Mail Transfer Protocol (SMTP) and how Simple Mail Transfer Protocol (SMTP) works. To learn the communication process between Simple Mail Transfer Protocol (SMTP) Client and Server,