What Is Simple Mail Transfer Protocol(SMTP)? And Fundamentals Of SMTP.

What Is Simple Mail Transfer Protocol (SMTP)? And Fundamentals Of SMTP.

Simple Mail Transfer Protocol (SMTP) is a Transmission Control Protocol/Internet Protocol protocol (TCP/IP) used in sending and receiving e-mails. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, Post Office Protocol version 3 (POP3) or Internet Message Access Protocol (IMAP), that let the user save messages in a server mailbox and download them periodically from the server.

In other words, users typically use a program that uses Simple Mail Transfer Protocol for sending e-mail and either Post Office Protocol version 3 or Internet Message Access Protocol for receiving e-mail. On Unix-based systems, sendmail is the most widely-used Simple Mail Transfer Protocol (SMTP) server for e-mail. A commercial package, Sendmail, includes a Post Office Protocol version 3 (POP3) server. Microsoft Exchange includes a Simple Mail Transfer Protocol server and can also be set up to include POP3 support.

Simple Mail Transfer Protocol usually is implemented to operate over Internet port 25. An alternative to Simple Mail Transfer Protocol that is widely used in Europe is X.400. Many mail servers now support ESMTP (Extended Simple Mail Transfer Protocol), which allows multimedia files to be delivered as e-mail.
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Simple Mail Transfer Protocol
Simple Mail Transfer Protocol (SMTP) Fundamentals
Simple Mail Transfer Protocol is an application layer protocol. The client who wants to send the mail opens a Transmission Control Protocol connection to the SMTP server and then sends the mail across the connection. 

The Simple Mail Transfer Protocol server is always on listening mode. As soon as it listens for a Transmission Control Protocol connection from any client, the SMTP process initiates a connection on that port (25). After successfully establishing the Transmission Control Protocol connection the client process sends the mail instantly.

Simple Mail Transfer Protocol (SMTP) Protocol

The Simple Mail Transfer Protocol (SMTP) model is of two types:
  • End-to-end method
  • Store-and-forward method
  • The user agent (UA)
  • Local MTA

The end to end model is used to communicate between different organizations whereas the store and forward method are used within an organization. 

A Simple Mail Transfer Protocol (SMTP) client who wants to send the mail will contact the destination’s host SMTP directly in order to send the mail to the destination. The Simple Mail Transfer Protocol server will keep the mail to itself until it is successfully copied to the receiver’s SMTP.

The client-SMTP is the one which initiates the session let us call it as a client-SMTP and the server-SMTP is the one which responds to the session request and let us call it as receiver-SMTP. The client-SMTP will start the session and the receiver-SMTP will respond to the request.

Model of Simple Mail Transfer Protocol (SMTP) system

In the Simple Mail Transfer Protocol model user deals with the user agent (UA) for example Microsoft Outlook, Netscape, Mozilla etc. In order to exchange the mail using TCP (Transmission Control Protocol), MTA (Mail Transfer Agent) is used. 

The users sending the mail do not have to deal with the Mail Transfer Agent it is the responsibility of the system admin to set up the local Mail Transfer Agent. 

The Mail Transfer Agent maintains a small queue of emails so that it can schedule repeat delivery of mail in case the receiver is not available. The Mail Transfer Agent delivers the mail to the mailboxes and the information can later be downloaded by the user agents.

Both the SMTP(Simple Mail Transfer Protocol)-client and MSTP (Multiple Spanning Tree Protocol)-server should have 2 components

Communication between the sender and the receiver : The senders, user agent prepare the message and send it to the Mail Transfer Agent. The Mail Transfer Agent functioning is to transfer the mail across the network to the receivers Mail Transfer Agent.


E-Mail is sent by a series of request and response messages between the client and a server. The message which is sent across consists of a header and the body. 

A null line is used to terminate the mail header. Everything which is after the null line is considered a body of the message which is a sequence of ASCII characters. The message body contains the actual information read by the receipt.


The user agent at the server side checks the mailboxes at a particular time of intervals. If any information is received it informs the user about the mail. 

When a user tries to read the mail it displays a list of emails with a short description of each mail in the mailbox. By selecting any of the mail user can view its contents on the terminal.

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What Is Simple Mail Transfer Protocol(SMTP)? And Fundamentals Of SMTP. What Is Simple Mail Transfer Protocol(SMTP)? And Fundamentals Of SMTP. Reviewed by Redbuddy on January 20, 2019 Rating: 5

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