What is a SMTP Server?

Email Marketing

What is a SMTP Server?

What is a SMTP Server?

Email is among the widely used communication channel, and most internet system uses Simple Mail Transfer Protocol SMTP to transfer email from their server to the recipient’s server.

It’s a standard communication protocol used by mail servers and other message transfer agents and can impact the deliverability of your campaigns.

This guide will discuss what SMTP is and how it works in the email marketing landscape.

What is the Simple Mail Transfer Protocol (SMTP)?

The Simple Mail Transfer Protocol (SMTP) is a technical standard for transmitting electronic mail (email) over a network. Like other networking protocols, SMTP allows computers and servers to exchange data regardless of their underlying hardware or software. Just as the use of a standardized form of addressing an envelope allows the postal service to operate, SMTP standardizes the way email travels from sender to recipient, making widespread email delivery possible.

SMTP is a mail delivery protocol, not a mail retrieval protocol. A postal service delivers mail to a mailbox, but the recipient still has to retrieve the mail from the mailbox. Similarly, SMTP delivers an email to an email provider's mail server, but separate protocols are used to retrieve that email from the mail server so the recipient can read it.

What is a protocol?

A protocol consists of a set of rules and procedures which govern the exchange of data between two or more devices. Protocols define how data transmission will occur between electronic devices such as computers. They set the standard procedures for communication and the exchange of information.

The International Organization for Standardization established the Open Systems Interconnection (OSI). One such widely used Internet protocol lays down the standard for communication over different networks. The model divides the process of data transmission into a series of seven layers.

Important internet protocols are TCP/IP, HTTPS, DNS, and SMTP.

What is an SMTP server?

An SMTP server is a mail server that can send and receive emails using the SMTP protocol. Email clients connect directly with the email provider's SMTP server to begin sending an email. Several different software programs run on an SMTP server:

  • Mail submission agent (MSA): The MSA receives emails from the email client.
  • Mail transfer agent (MTA): The MTA transfers emails to the next server in the delivery chain. As described above, it may query the DNS to find the recipient domain's mail exchange (MX) DNS record if necessary.
  • Mail delivery agent (MDA): The MDA receives emails from MTAs and stores them in the recipient's email inbox.

Common SMTP server providers & settings

SMTP ProviderURLSMTP Settings

How does the SMTP work?

The Transmission Control Protocol (TCP) is the primary connection for communication between the mail sender and the mail receiver. In SMTP, the mail sender sends the data in command strings over this reliable ordered data stream channel.

The SMTP client, the initiating agent, sender, or transmitter, initiates the communication session. It issues the command strings and opens the session for corresponding responses from the SMTP server, which involves the listening agent or receiver. Zero or more SMTP transactions may be there in a course.

Working of SMTP (Simple Mail Transfer Protocol)

Usually, an SMTP email transaction follows four command or reply sequences:

HELO/EHLO command

It tells the email server that the client wants to start the mail transaction. The client mentions its domain name after this command.

MAIL command

It lays down the bounce address/return address, defining the return or reverse paths.

RCPT command

It specifies the recipient of the message. The sender’s envelope contains the addresses of the recipients, to which the RCPT command can be issued multiple times for each recipient.


It shows where the content of the message starts, as opposed to its envelope. An empty line separates the message header and body in the message’s text.

DATA is not just one command but a group of commands in which a server has to reply twice

  • First, the server acknowledges the message and replies with its readiness to take the message.
  • Then after completing the end-of-data sequence, it either accepts or rejects the entire message.

Apart from the reply of the DATA command, the server can reply in a positive way (2xx reply codes) or a negative way.

The negative responses can further be permanent (5xx codes) or transient (4xx codes).

If a server sends ‘reject,’ then it is a permanent failure, and the client needs to send a bounce message to the respective server. On the other side, a ‘drop’ is a positive reply in which the message is discarded instead of delivered.

What is an SMTP envelope?

The SMTP “envelope” is the set of information that the email client sends to the mail server about where the email comes from and where it is going. The SMTP envelope is distinct from the email header and body and is not visible to the email recipient.

What port does SMTP use?

In networking, a port is a virtual point where network data is received; think of it as the apartment number in the address of a piece of mail. Ports help computers sort networking data to the correct applications. Network security measures like firewalls can block unnecessary ports to prevent the sending and receiving of malicious data.

Historically, SMTP only used port 25. Today, port 25 is still in use for SMTP, but it can also use ports 465, 587, and 2525.

  • Port 25 is most used for connections between SMTP servers. Firewalls for end-user networks often block this port today, since spammers try to abuse it to send large amounts of spam.
  • Port 465 was once designated for use by SMTP with Secure Sockets Layer (SSL) encryption. But SSL was replaced by Transport Layer Security (TLS), and modern email systems, therefore, do not use this port. It only appears in legacy (outdated) systems.
  • Port 587 is now the default port for email submission. SMTP communications via this port use TLS encryption.
  • Port 2525 is not officially associated with SMTP, but some email services offer SMTP delivery over this port in case the above ports are blocked.

Difference between SMTP, IMAP, & POP3

The Internet Message Access Protocol (IMAP) and Post Office Protocol (POP) are used to deliver the email to its final destination. The email client has to retrieve the email from the final mail server in the chain to display the email to the user. The client uses IMAP or POP instead of SMTP for this purpose.

To understand the difference between SMTP and IMAP/POP, consider the difference between a plank of wood and a rope. A length of wood can be used to push something forward, but not pull it in. A rope can pull an item, but cannot push it. Similarly, SMTP “pushes” email to a mail server, but IMAP and POP “pull” it the rest of the way to the user's application.

What is Extended SMTP (ESMTP)?

Extended Simple Mail Transfer Protocol (ESMTP) is a version of the protocol that expands upon its original capabilities, enabling the sending of email attachments, the use of TLS, and other capabilities. Almost all email clients and email services use ESMTP, not basic SMTP.

ESMTP has some additional commands, including “EHLO”, an “extended hello” message that enables the use of ESMTP at the start of the connection.


SMTPs are essential for sending and receiving emails. However, as an email marketer, you need to choose and configure the SMTP service providers that suit your requirements.

At Sapnaaz, we provide easy SMTP setup and integration with any SMTP servers you might like to send your email marketing campaigns. Feel free to reach out to the Sapnaaz team to learn more or check out our SMTP setup service page.


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