What is the SMTP server? Guide for Marketers

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If you use transactional emails on your website or do any email marketing efforts, you may have heard the term SMTP and wondered, what is SMTP and what does it have to do with me?

This protocol is of vital importance for marketers as it directly impacts the emails they send to their customers and their deliverability.

Although SMTP is quite a technical topic, and all of this may sound a bit for engineers, don't worry, this guide will teach you everything you need to know, but from a less technical point of view so you can better understand how SMTP can impact your marketing strategy.

All the faces of SMTP

While the acronym SMTP actually refers to the communication protocol used by email servers, it is also used as:

Read on to find out the differences between the three.

SMTP protocol

SMTP primarily refers to the set of rules that mail servers use to transfer electronic messages over the Internet.

To be more exact, the protocol works as follows:

  1. The sender and recipient of the message are specified
  2. A request is sent to both sender and recipient to verify that both addresses exist
  3. The message is transferred from the sender to the recipient

Steps to send messages via SMTP protocol (for those who really want to learn)

If you're a marketer, you might not be interested in the details of how this protocol works, but if you want to shine at the next team meeting, here's a more detailed rundown of the process:

A few important acronyms:

  • MUA (Mail User Agent) - Email client (eg Gmail, Yahoo, Outlook)
  • MSA (Mail Submission Agent) - Computer software that runs on an SMTP server, receives messages from a MUA and checks for any errors before transferring it to an MTA.
  • MTA (Mail Transfer Agent): software that verifies the MX record of the recipient's domain to decide how to proceed with the transfer of the message (whether to send it to another MTA or to an MDA)
  • MX record (Mail Exchanger Record): it is a record that specifies from which server the recipient's MUA can retrieve the message.
  • MDA (Mail Delivery Agent): software that stores messages for retrieval by MUAs


These are the 6 main steps that make up the transfer process for SMTP:

  1. The email is sent by a MUA to the MSA of a mail server
  2. The message is transferred to the server's MTA (the MTA and MSA are usually hosted on the same SMTP server)
  3. The MTA verifies the MX record of the recipient's domain and passes the message to another MTA (this step can occur multiple times until the correct receiving server receives the message)
  4. The message is transferred to the MDA, which saves the messages in the proper format for retrieval by the MUA.
  5. The MUA requests the message from the MDA (usually with POP3 or IMAP)
  6. The message is sent to the inbox of the receiving MUA


To give you a clearer overview, here is a graphic showing the process in action:

SMTP server



SMTP relay

Surely in your search for email marketing you have come across the term SMTP relay more than once. It is a more technical term that refers to the act of transferring an email to the appropriate destination server.

However, in practice, the term SMTP relay more often refers to the actual servers that emails pass through before reaching their destination, such as a mail processing and distribution center. In this case, you can think of "SMTP relay" as a synonym for SMTP server.

SMTP server

An SMTP server is simply the server that receives, handles, transfers or stores messages according to the rules of SMTP (the communication protocol).


SMTP servers and deliverability

Now that you understand a little better (I hope) what SMTP means, let's get down to what really interests you: the deliverability or "deliverability" of e-mail.

The SMTP server you use to send your emails is associated with a specific IP address. This IP address is directly related to deliverability as it is the address that ISPs use to verify the reputation of its sender.

Due to the nature of their service, the free SMTP servers configured by the classic email providers (Gmail, Yahoo, Outlook, etc.) are not adapted to handle the sending of mass emails.

The reason for this is that their IP addresses are not closely monitored, which means that you could be sending emails from the same server as a spammer. Therefore, if you send mass emails through classic email providers like the ones already mentioned, you end up in the (dreaded) Spam folder.

What is the solution to this SMTP server problem?

If you want to achieve the best possible deliverability for your bulk emails, you should use a professional SMTP relay service that has been expressly configured for the purpose of delivering bulk emails.

At Sendinblue, all of our servers are built on a solid and trusted infrastructure that allows us to send millions of emails every day for our clients.

Because we maintain strict control over all of our IP addresses, our users can benefit from excellent deliverability for both marketing and transactional emails.

If you need even more control over the reputation of the sender of your emails, Sendinblue also offers dedicated IP addresses to our senders with the highest volume of sending.

This means you don't have to worry about anyone else's sending behavior as it won't affect the "deliverability" or deliverability of your emails.

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