Email best practices

What is an SMTP Relay and why do we use it?

All the information you ever needed on SMTP Relay. Find out more about what SMTP Relay is and why it's an efficient solution for your email needs.

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Emailing is hard. It might look easy at first – just write something smart, press 'Send' and wait, right? But mastering the ins and outs of emailing is actually not that simple. The world of email is full of complicated words like email deliverability or relay servers, confusing acronyms like SMTP, MTA, or IMAP, and detailed metrics that need to be understood.

Setting up contact lists and creating your first newsletter are great starting points, but if you’re looking to take the next step in your understanding of email marketing, then you should probably take a closer look at SMTP relays.

SMTP stands for Simple Mail Transfer Protocol, and is essentially the backend system that helps you and your company send, receive, and relay messages between email senders and receivers. Let’s take a look at what it is and how it works.


An SMTP relay is a protocol that allows email to be transmitted through the internet, from one server to another, for delivery. It was first created in 1982 and continues to be the internet standard that is widely used today.

An SMTP relay is an email relay service that basically works in two steps:

  1. It receives the outgoing mail from the sender (that is, your amazing Groundhog Days email campaign).

  2. It delivers it to the recipient’s local post office, another SMTP server.

Simple, right?

Hold on, but what is 'email relay'?

OK, so maybe not that simple yet. To break this down a bit more, let’s imagine the journey that your normal snail mail may take to get to its destination:

Email relay is the process of transmitting an email message from one server to another. In the picture above, the local post offices would be the SMTP servers and the email transfer that happens between them is what we call ‘relaying’.

So for example, when you send out your latest campaign ‘Cute puppies looking for a foster home’, your company’s SMTP server relays your email to the server owned by your recipient. But if you were to send that campaign to someone with your same domain, there would be no ‘email relay’, as your SMTP relay server wouldn’t need to transfer the email to a different SMTP server.

Sending through an SMTP server with an email service provider

So what does this protocol look like when it comes to an email service provider like Mailjet? Getting the most out of your own SMTP relay server is not easy, so most businesses that need to send mass email to their customers use SMTP relay for ease of maintenance and added analytics insights.

Sending through an email service provider via an SMTP relay saves companies from having to run their own mail server. As you can see in the diagram below, the business or sender creates the email and their server sends it to Mailjet’s SMTP server to prepare and send it out to recipients.

In order to combat spam, a majority of webmail providers and email clients (i.e. Gmail, Outlook, Yahoo, etc.) put a limit on how many emails you can send to different recipients per day. As businesses who need to communicate en mass with their audience would often exceed this limit, they will require the services of an enterprise level email sending platform – like Mailjet.

An SMTP relay provider can help businesses and organizations deliver large volumes of email without getting them mislabeled as spam or running up against small sending limits.

Email service providers invest a lot of resources into building their own email infrastructure to handle large volumes and work closely with the major internet service providers (ISPs) and webmail providers to improve email deliverability and deliver these emails straight to the recipient's inbox.


Send mass email with Mailjet’s free SMTP relay

Send large volumes of email using Mailjet’s free SMTP service. Benefit from great email deliverability and advanced analytics, and improve your email ROI.

Behind the scenes: SMTP server tracking

There’s an added layer of value to sending through an email service provider. With Mailjet, before our SMTP servers send an email, our system automatically adds link trackers in the body of your message. This then allows you, as the user, to properly track opens and clicks after an email has been received.

Mailjet also translates feedback from ISPs (Gmail, Yahoo, AOL, etc.), since each one communicates in its own way. Our service saves developers time by converting this into an easily identifiable response, displaying whether an email has been delivered or has bounced. These metrics make it easier for you to identify issues that might impact your email deliverability.

Want to know if a third-party SMTP relay service is the choice for your company? Our Email Infrastructure Handbook helps you understand the pros and cons of homebuilt and outsourced email infrastructure and find the best fit for you

Infrastructure guide

The Email Infrastructure Handbook: Homebuilt vs Outsourced Guide

Download our email infrastructure guide here

Choosing an SMTP relay port

Aha! Another tricky aspect of the email world. To understand what SMTP ports are and how they work, we need to take a step back and see what happens when computers communicate with each other on the internet.

Let’s say you are trying to reach In this case, the Domain Name System (DNS) is converting this to the actual IP address that is hidden behind the name of the site. In Mailjet’s case, this is You probably could remember 4-5 IPs like ours, but who can actually remember more, or really…who would want to?

An SMTP port is one that is meant to be used for SMTP connections. Today, the most common SMTP ports are 25, 465, 587, or 2525. This doesn’t mean that they are the only ones, though. These few ports are the most used ones for these types of connection, and because of that they are almost always open, which means you should be able to reach your destination.

Choosing the right SMTP relay port to use is actually an important consideration when it comes to SMTP. So much so that we devoted an entire blog to this already. If you're trying to decide which port to use, be sure to read our article Which SMTP Port Should I Use? to see which one is best for you.

Wrapping up

Ultimately, SMTP relay makes our lives as marketers much simpler by handling all of the heavy lifting in the backend so that we can spend more time crafting content and building out our contact lists.

Still want to learn more about SMTP relays? Check out this episode of Email Explained. Our Sr. Technical Account Manager gives us the 101 of what you need to know about SMTP Relay.

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