February 1, 2018

What is SMTP (Servers, Relays, and Protocol): A Guide for Marketers

Reading time about 7 min

If you use transactional emails on your website or do any kind of email marketing, you’ve probably heard the term SMTP — but you might be wondering, what is SMTP and what does it have to do with me?

This protocol is important for marketers because it directly impacts the emails you send and their deliverability.

If that all sounds a little daunting, don’t worry — although SMTP is a pretty technical subject, this guide will teach you what you need to know from a less technical standpoint so you can better understand how it impacts your email marketing.

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The many faces of SMTP

While the acronym SMTP actually refers to the communication protocol used by mail servers to transmit electronic messages (i.e. emails) across the internet, the term has also been used interchangeably to mean the following:

  • SMTP (protocol)
  • SMTP relay
  • SMTP server

Read on to learn the differences between the three.

SMTP Protocol

As we mentioned above, SMTP primarily refers to the set of rules that mail servers use to transfer emails over the internet from the sender’s SMTP server to the proper receiving SMTP server where the message can be accessed by the recipient through their email inbox client.

More concretely, the protocol works like this:

  • The sender and recipient of the message are specified
  • A request is sent to both the sender and recipient to verify they exist
  • The message is transferred from the sender to the recipient

Steps for transferring messages over the internet according to SMTP (for those who really want to know)

If you’re a marketer, you might not be interested in the minute details of how SMTP works in practice. But just in case you want to shine in the next meeting you have with your tech team, here is a more detailed look at the process:

Acronyms related to SMTP:

  • MUA (Mail User Agent) – Email client (e.g. Gmail, Yahoo, Outlook)
  • MSA (Mail Submission Agent) – Computer software run on an SMTP server that receives messages from a MUA and checks for any errors before transferring to a MTA.
  • MTA (Mail Transfer Agent) – Software that checks recipient domain’s MX record to decide how to continue transferring the message (either to another MTA, or an MDA)
  • MX Record (Mail Exchanger Record) – Is a resource record that specifies from which server the recipient MUA for that domain can retrieve the message.
  • MDA (Mail Delivery Agent) – Software that stores messages for batch retrieval for MUAs

Now that you’ve got the SMTP vocabulary down, it’s time to talk about how they all fit together to create the protocol for transferring electronic messages over the internet.

Here are the 6 main steps that comprise the electronic message data transfer process for SMTP:

  1. Email is submitted by a MUA to a mail server’s MSA
  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 checks the MX record of the recipient domain and transfers the message to another MTA (this step can occur multiple times until the message is received by the proper receiving server)
  4. The message is handed off to the MDA, which saves messages in the proper format for retrieval by the receiving MUA
  5. The receiving MUA requests the message from the MDA (usually with POP3 or IMAP)
  6. The message is delivered to the receiving MUA‘s inbox

To give you a more clear visual — here is a graphic that shows the process in action:

SMTP server transfer steps

SMTP Relay

In your reading on email marketing, you might have also come across the term SMTP relay. This is a more technical term that refers to the act of routing an email to the proper destination SMTP server on behalf of a sender. It usually relates specifically to the use of a special server set up by a bulk email service (ahem… like transactional emails sent from a website or web application to recipients with enhanced deliverability.

In practice though, the term SMTP relay more often refers to the actual SMTP servers through which emails pass before arriving at their destination like a mail processing and distribution center. In this case, you can think of “SMTP relay” as a synonym for SMTP server.

SMTP Server

As we mentioned earlier, SMTP is the communication protocol used to securely transmit emails over the internet. So what does server mean?

“Server” is just a fancy word for a computer. But, servers function quite differently from your normal personal computer:

  • Personal computers (e.g. your laptop or desktop) allow users to store different files locally using an operating system like Windows or Mac OS, as well as perform other tasks using applications or client web applications.
  • Servers, on the other hand, are computers that operate only to store, process, or transfer data between other servers or to personal computers (clients) through requests in order to produce a specific functionality for the client software making the request.

In the case of email, the client software is either the application you use to access your email (e.g. Mozilla Thunderbird or Microsoft Outlook) or webmail inbox provider (e.g. Gmail or Yahoo) that you use to access your emails from the Mail Delivery Agent. The servers are all of the other machines involved in the SMTP process (MDA, MSA, MTA).

These servers run programs that wait for SMTP commands from SMTP clients (the sender). Once they receive a command (or request) from the SMTP client, the server establishes a connection with that client and begins routing the electronic message to the proper recipient according to the rules stipulated by SMTP (the protocol).

Therefore, any server that receives, handles, transfers, or stores electronic messages being transferred through SMTP can be considered an SMTP server.

However, in the case of email marketing, SMTP server mostly refers to the general service provided by bulk email sending platforms. This is because companies like Sendinblue have large amounts of dedicated servers that are specially configured in a way that supports high-volume sending for end users (unlike the SMTP servers used to send your typical email from your Gmail account).

SMTP Servers and Deliverability

Now that you have a better understanding of what SMTP means, let’s get to what you’re really interested in: email deliverability.

The SMTP server that you use to send out your emails is associated with a specific IP address. This address is directly tied to your deliverability because it is this address that ISPs use to check your sender reputation.

As we mentioned earlier, due to the nature of their service, the SMTP servers set up by classic email providers (Gmail, Yahoo, Outlook, etc.) for sending personal emails are not adapted to handle the sending of bulk emails.

The reason for this is that their IPs are not closely monitored. That means you could be sending emails from the same server as a known spammer. If you try to send bulk emails through classic email providers like this, you are guaranteed to end up in the 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’ll need to use a professional SMTP relay service that has been set up expressly for the purpose of delivering bulk emails.

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

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

If you require even more control over your sender reputation, Sendinblue also offers dedicated IP addresses to our higher-volume senders. This means you don’t have to worry about the sending behavior of anyone else affecting your deliverability.

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