Simple Mail Transfer Protocol | SMTP Protocol

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Application Layer Protocols-


Important application layer protocols are-



  1. Domain Name Service (DNS)
  2. Hyper Text Transfer Protocol (HTTP)
  3. Simple Mail Transfer Protocol (SMTP)
  4. Post Office Protocol (POP)
  5. File Transfer Protocol (FTP)


In this article, we will discuss about Simple Mail Transfer Protocol (SMTP).


Simple Mail Transfer Protocol-


  • SMTP is short for Simple Mail Transfer Protocol.
  • It is an application layer protocol.
  • It is used for sending the emails efficiently and reliably over the internet.




  • SMTP server is always on a listening mode.
  • Client initiates a TCP connection with the SMTP server.
  • SMTP server listens for a connection and initiates a connection on that port.
  • The connection is established.
  • Client informs the SMTP server that it would like to send a mail.
  • Assuming the server is OK, client sends the mail to its mail server.
  • Client’s mail server use DNS to get the IP Address of receiver’s mail server.
  • Then, SMTP transfers the mail from sender’s mail server to the receiver’s mail server.



While sending the mail, SMTP is used two times-

  1. Between the sender and the sender’s mail server
  2. Between the sender’s mail server and the receiver’s mail server




To receive or download the email,

  • Another protocol is needed between the receiver’s mail server and the receiver.
  • The most commonly used protocols are POP3 and IMAP.


Characteristics of SMTP-


  • SMTP is a push protocol.
  • SMTP uses TCP at the transport layer.
  • SMTP uses port number 25.
  • SMTP uses persistent TCP connections, so it can send multiple emails at once.
  • SMTP is a connection oriented protocol.
  • SMTP is an in-band protocol.
  • SMTP is a stateless protocol.


Important Points-




SMTP is a pure text based protocol.


  • SMTP can only handle the messages containing 7 bit ASCII text.
  • SMTP can not transfer other types of data like images, video, audio etc.
  • SMTP can not transfer executable files and binary objects.
  • SMTP can not transfer the text data of other languages like French, Japanese, Chinese etc.

(since they are represented in 8 bit codes)




MIME extends the limited capabilities of email.


As the name suggests,

  • Multipurpose Internet Email Extension (MIME) is an extension to the internet email protocol.
  • It extends the limited capabilities of email by enabling the users to send and receive graphics, audio files, video files etc in the message.
  • MIME was specially designed for SMTP.




SMTP is a stateless protocol.


This is because-

  • It does not maintain the state of its clients.
  • If an email is asked to be sent twice, then SMTP server resends it without saying that the email has already been sent.




We can not use SMTP at the receiver’s side.


This is because-

  • SMTP is a push protocol.
  • At receiver’s side, a pull protocol like POP3, IMAP is needed.
  • Receiver periodically checks if he has any mail from his mail server.




Sender and receiver can not run SMTP between their machines.


This is because-

  • Machines can not always be ON.
  • So, the functionality has been divided between the client and the mail server.
  • The mail server receives the mail on behalf of its client and manages the mail box of the client.




SMTP is not suitable for client authentication.


This is because-

  • SMTP does not require authentication.
  • It allows anyone on the Internet to send emails to anyone or even to a large group of people.
  • SMTP Auth short for SMTP Authentication has been provided for authentication.


To gain better understanding about SMTP Protocol,

Watch this Video Lecture


Next Article- Post Office Protocol | POP


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Simple Mail Transfer Protocol | SMTP Protocol
Article Name
Simple Mail Transfer Protocol | SMTP Protocol
SMTP is short for Simple Mail Transfer Protocol. SMTP Protocol is an application layer protocol. SMTP in Computer Networks is a stateless, connection oriented, push, in-band protocol. SMTP uses TCP port 25 at the transport layer.
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Gate Vidyalay
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