SMTP Stands for Simple Mail Transfer Protocol.

  1. The TCP/IP protocol that supports e-mail on the Internet called the simple mail transfer protocol(SMTP).
  2. SMTP is a simple ASCII (American Standard Code for Information Interchange) protocol.
  3. Once a TCP connection between a sender and port 25 of the receiver is established, the sending machine operates as the client and the receiving machine acts as a server. The client then waits for the server to talk first. The server sends a line of text to give it an identity and announces its willingness/unwillingness to receive mail. if the server is not prepared the client to release the connection and tries again later.
  4. Electronic mail(e-mail) is the most popular among Internet service for sending messages from one person to another.
  5. Simple Mail Transfer Protocol(SMTP) is the standard mechanism mail on the internet.

Problems in SMTP

  1. Some older implementations are not capable of handling messages longer than 64 kb.
  2. If the client and server have different time-outs, then one of them may give up when the other is still busy.
  3. In rare situations, infinite maelstroms can be triggered.

Extended SMTP(ESMTP)

Some of these problems can be solved by using the extended SMTP(ESMTP) defined in RFC 1425.

SMTP commands

HELO command: Commands that initiate a new protocol session Between Client & Server.
EHLO command: The EHLO Command Request them to respond with any optional SMTP extension it supports.
MAIL From: command to initiate sending an email message or to identify the sender.
RCPT: Identify the intended Recipient.
DATA: Command Indicating the short of Transmission of the email message. The last message is “.” as a termination character to signify the end of the mail.
RESET command: This command used to reset the connection if encounter an error.
NOOP: Empty message like ping to check the responsiveness of other End.
QUIT: Terminates the protocol session.
MTA: Responsible for Transfering and Routing an email message from the sender’s computer to the recipient computer.

Example of Response

Positive condition response
211 – System status
214 – Help
220 – Service ready
221 – Service closing
Transient Negative completion reply
421 – Service NA
450 – Mailbox NA
451 – Command aborted
Permanent Negative completion Reply
500 – Syntax Error
554 – Transaction Failed

Advantage of SMTP

Simple to use

SMTP provides the simplest form of communication between one person to another via email messages.

Quick Email Delivery

Since the SMTP server is developed on the simplest platform, email messages can be sent easily and quickly.


It provides reliability in the context of outgoing messages.

Working of SMTP

Composition of Mail

A user sends an e-mail by creating an electronic mail message using a mail user agent (MUA). A mail user agent is a program used to send and receive mail.

The message consists of two parts: body and header.

Body – The body is the body of the message while the header contains information such as the address of the sender and the recipient.

Header – The header also contains descriptive information such as the subject of the message.

Submission of Mail

After writing an email, the mail client then sends the ready e-mail to the SMTP server by using SMTP on TCP port 25.

Delivery of Mail

An e-mail has two parts: the recipient and the domain name ie service provider. For example, [email protected], where Rahul is the recipient’s name and “” is the domain name.

If the recipient’s email address domain name is different from the sender’s domain name, the MSA will send mail to the mail transfer agent (MTA). To relay email, the MTA will get the target domain. It checks MX records from the domain name system to obtain the target domain. The MX record contains the domain name and the IP address of the recipient’s domain. Once recorded, the MTA connects to the Exchange server to relay the message.
Receipt and Processing of Mail: Once an incoming message is received, the Exchange server distributes it to the incoming server (mail delivery agent) which stores the e-mail where it waits for the user to retrieve it.

Access and Retrieval of Mail: Emails stored in MDA can be retrieved using MUA (Mail User Agent). MUA can be accessed using login and password.

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