What is SMTP Server?

Photo of author

By Sina Nasiri

Have you ever thought about how an email is sent, what are the commands, and the devices working behind the scene to make communication through emails possible? From the user’s standpoint, everything looks quite simple when they send an email. It doesn’t matter if the person is sitting on the other side of the office or he’s sitting on the other side of the world. The mail will reach them instantly, but how? Well, the answer lies with a complex system, which allows mails to be sent over the internet with the help of an SMTP server.

SMTP stands for Simple Mail Transfer Protocol, and it works as a bridge between the sender and the receiver. When you click on the send button, your email client, such as Gmail, Yahoo, etc., will connect to the mail server. Then the mail server is connected to the other mail servers, and it relays the mail which was sent by you and thus, allowing your mail to be sent to the receiver in no time.

The mail servers are designed to perform only mailing services. No other form of function or any other service will take place on mail servers. Well, this was just the definition of SMTP servers; if you want to know more about how an SMTP server works and what the commands it uses, you need to read this article till the end. So, let’s begin.

Working of SMTP Server 

The SMTP server uses the Transmission Control Protocol (TCP) connection between the mail sender and the mail receiver. In SMTP, when you are sending data over mail, it gets divided in the form of command strings and transferred via TCP, which is said to be an ordered data stream channel on the network.

There are two primary components of an SMTP client, and each one has its own purpose in the mail transfer. The first component is the initiating agent, and the second is the sender, which is also used as a transmitter during the communication sessions. The initiating agent will number the command strings. Also, it does open the session for corresponding responses from the SMTP server. These sessions require the listening agent or receiver. In an SMTP transaction, there could be more than one or zero transactions taking place side by side.

There are usually four commands which are used for the successful transmission of the mail or the reply sequence; these four commands are:

  • HELO/EHLO: This command will notify the email server that the client wants to start the mail translation. In this command, the client enters their domain name once this command has been sent to the server. For example, “HELO email.gmail.com.”
  • MAIL: This will provide the server with the bounce address/return address. Thus, the server knows the return path or the reverse path of the mail.
  • RCPT: In this command, the client enters the recipient’s address for the mail. In the envelope sent by the client, a recipient’s address is written. The RCPT command can be issued multiple times for each specific recipient. This command comes quite handy when you are broadcasting mail and sending it to everyone in the office.
  • DATA: This command will mark out the starting point of the contents which are present in the message. An empty line is used to separate the message header and the body in which the text of the mail is written. But you need to keep one thing in mind: the DATA is not just one command, but it is a group of commands, which makes the server reply twice.

First, the server acknowledges the mail and responds almost instantaneously to accept the mail. After that, the server processes the end-of-data sequence, in which it decides whether to accept the mail or reject it altogether.

In addition to the reply part of the DATA command, the server also comes with the functionality to reply in a positive way, such as 2XX response status codes, or it could reply in a negative way. Furthermore, the negative reply is of two types: the first one is permanent 5xx error codes and the transients, which are represented as 4xx codes.

Note:- When a mail server sends out the reject response, it becomes a permanent failure, and then the sender of the mail has to send a bounce message to that mail server. On the other hand, a drop is said to be a positive reply by the server, and it means a message is discarded instead of being delivered to the recipient.

Different Types of SMTP Server Models

There are two types of SMTP server models, and each of them is described below: 

1. End-to-End Method

The end-to-end method of SMTP allows transmission of the data in which only the sender of the data and the receiver are able to read the message. The data which is to be sent via mail is encrypted from the sender’s machine. Only the recipient who is mentioned to receive the mail gets access to read and view the information. During the transfer of the mail, nobody can dampen the information which is present inside the mail. Thus, the end-to-end encryption method provides the highest form of security and confidentiality to your email communications.

For end-to-end methods to work correctly, both the receiver and the sender need to have cryptographic keys. In this method, both the parties have a public key and a private key. These keys are generated only once when the client is creating their first encrypted email account. The sender of the mail encrypts the mail locally on their device by using the public key of the recipient. The receiver, after getting the mail, decrypts it by using a private key.

2. Store-and-Forward Method

The second type of SMTP server model is based on the store-and-forward method, and it is used when a client and the recipient are not connected directly. The store and forward technique stores the message which is sent by the client. The message is stored in an intermediary device which is generally a server.

After that, a server locates the destination device from its subscriber’s database and initiates a connection. Once the connection is established, the data which contains the message will be sent to the recipient. A lot of your emails are sent by using this method as most of the time, you and the recipient of the mail are geographically distant from one another.

Configuring SMTP Server 

Before you think about sending mail directly from the server, you first need to make a connection of your device with the SMTP server. The good thing about SMTP servers is they are pretty easy to configure. All you have to do is open your mailing software and add the correct SMTP parameters in the settings.

Keep in mind that when you are setting up a free SMTP service like Gmail, Hotmail, etc. You are not guaranteed to have a 100% successful mail sending rate, especially if you are sending mails to a massive number of recipients. Thus, we recommend you to go with a professional SMTP service such as TurboSMTP or any other which you prefer. Given below is the standard procedure for setting up the SMTP server in four simple steps:

  1. First, go to your mail settings which are usually present in the tools menu. 
  2. After that, you need to choose the outgoing server SMTP settings.
  3. Click on the add button to set up a new SMTP server. Now you will see a popup on your screen.
  4. All you have to do is fill up the information required in the popup, and you are good to go.

Below are all the fields that you need to fill within the popup:

  • Description: Here, you need to give a name to your SMTP server; this name will be used to identify the SMTP server.
  • Server Name: Now, you need to put the original SMTP server’s specification. You can quickly locate it on the consulting page of your SMTP provider.
  • Port: You need to specify the port to which your SMTP server will be attached to your router. In most cases, SMTP works on port 25, but you can also use it with port number 587.
  • Connection Security: SMTP is not going to provide you with the encryption, so if you want to make your connection secure, you need to get STARTTLS or SSL/TLS extension; this will lead to a separate port for sending encrypted messages, emails, and communication.
  • Authentication: You can use a number of methods to authenticate the SMTP connection; you can use passwords, CRAM-MD5, and others, depending on your requirement.
  • User Name: Lastly, you need to provide the email address which you will be using for the SMTP.

10 Important SMTP Commands

Given below are the top 10 most crucial SMTP commands that are used by the server to send your email securely. Each command is used in the regular message transfer through the SMTP server.

  1. HELO: This is the first SMTP command, and it is sent to let the server know that the client wants to send an email, and this command is followed by a domain name.
  2. EHLO: This is an alternative command to give the server a signal to start the conversation from the client’s end. This command is also followed by the domain name.
  3. MAIL FROM: This command will notify the server that operations for sending the mail need to be performed now. In this command, the sender provides the source of the email address, and the actual email transfer will start.
  4. RCPT TO: Here, the client will provide the recipient’s address where the mail needs to be sent. If there is more than one recipient of the same mail, the command can be used multiple times.
  5. SIZE: This command will provide information to the SMTP server about the size of the attached email. Moreover, this command is also used to report the maximum size of a message that the server can accept.
  6. DATA: With this command, the client’s machine will initiate the transfer process of the content present in the mail. The general reply code of this command is 354 from the server end. This reply is actually permission which is sent to the client’s machine to start the transmission of the messages.
  7. VRFY: This is used to make the server check if the particular email address or the username of the recipient actually exists or not.
  8. TURN: This will change the roles of the client and the server, respectively, without needing to set up a new connection.
  9. RSET: This command is used to reset the SMTP connection to its initial state, and it will erase all the buffers and state tables both for the sender and to the receiver. This command only gets a positive response of 250. Also, it makes the SMTP server open up the connection and ready itself for the new message transfer.
  10. QUIT: The quit command is sent to request the server to terminate the SMTP session. When you are getting the response code 221, it means that the client or the sender has closed the SMTP connection. The receiver of the 221 response code needs to send back a reply 221: OK, and then the transmission channel will be closed completely.

Benefits of SMTP Server

Given below are some of the significant advantages of using an SMTP server for sending emails:

1. Simple 

SMTP is the simplest form of communication through email messages to multiple computers which are connected to the network. There are no hassles which the user needs to take care of when they are trying to send the mail to the recipient’s email address.

2. Quick Email Delivery

If the SMTP server is working as it should, the whole process of sending and receiving the mail can be done in no time.

3. Reliability 

If somehow, on the first try, the SMTP server is not successful in sending the mail to the particular address. Then it will keep resending it until the transmission is completed.

4. Ensures Emails Delivery

SMTP relay is a service that takes the role of the name resolution and reliable network connection by the DNS when it is delivering mail to the recipient. When the mails sent through SMTP reach the recipient’s email server, they are given a green signal to directly go into the recipient’s inbox.


SMTP server is a great way to send emails professionally and securely. Also, it helps you grow your business by reaching out to more potential customers.SMTP allows you to send more emails per day than any other mailing server. So it’s a must-have for new businesses that are thinking about growing their sales numbers through email marketing.

SMTP, even after so many years, has remained a gold standard for sending and receiving mails, and its supremacy is likely going to continue for many years to come.

People are also reading: 

Leave a Comment