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Email is sent and received using SMTP. IMAP or POP3 are occasionally used in conjunction with it (for instance, by a user-level programme) to handle message retrieval, whereas SMTP is generally used to transmit messages to servers for forwarding.

In this article, we will look more into SMTP according to the GATE Syllabus for (Computer Science Engineering) CSE. We will read ahead to find out more about it.

Table of Contents

What is SMTP?

The Simple Mail Transfer Protocol (SMTP) is a set of rules for communication that enables applications to send electronic mail over the internet. Based on email addresses, it is an application used to deliver messages to other computer users.

Between users using the same or other computers, it offers mail exchange, and it also supports:

  • One message may be sent to one or many recipients.
  • Messages can be sent using text, audio, video, or graphics.
  • The messages may also be transmitted through networks other than the internet.

Setting up communication rules among servers is the major usage of SMTP. The servers can identify themselves and state the type of communication they are attempting to carry out. They also provide a method for dealing with issues like a wrong email address. For instance, if the recipient address is incorrect, the receiving server will respond with some sort of error message.

Components of SMTP

The SMTP client and SMTP server have two components, the user agent (UA) and the mail transfer agent (MTA). The message is prepared by the UA, who also makes the envelope and inserts the message. This letter is sent across the internet by the MTA.

By including a relaying system, SMTP enables a more intricate system. More MTAs can be added, operating either as a client or a server to relay the email, rather than just having one MTA at the sending side and one at the receiving side.

The primary use of the mail gateway makes it possible to send emails to people using a relaying system without the TCP/IP protocol. A relay MTA that can receive emails is the mail gateway.

Working of SMTP

Composition of Mail

By creating an electronic mail message with an MUA, a user sends an email. An application called Mail User Agent is used to send or receive mail. The body and header are the two components of communication. The body of the communication is its main part, whereas the header includes information like the addresses of the sender as well as the recipient. The header also includes descriptive information like the message’s subject. In this case, the header mimics an envelope with the recipient’s address, while the message body resembles a letter.

Submission of Mail

The mail client then sends the finished email to the SMTP server using SMTP on the TCP port 25 after composing it.

Delivery of Mail

The recipient’s username and the domain name are the two components of an email address. For instance,, where the recipient’s login is “neeraj” and the domain name is “”

Mail will be sent to the Mail Transfer Agent if the recipient’s email address’s domain name differs from the sender’s domain name (MTA). The MTA will locate the target domain and relay the email there. To retrieve the target domain, it looks up the MX record from the Domain Name System. The IP address and the domain name of the recipient’s domain are listed in the MX record. MTA establishes a connection with the exchange server in order to relay the message after locating the record.

Receipt and Processing of Mail

Once the incoming message comes, the exchange server sends it to the mail delivery agent (incoming server), which archives the email and makes it available for user retrieval.

Access and Retrieval of Mail

Using MUA, one can retrieve the emails that were saved in MDA. MUA is accessible with a login and password.

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