What is an SMTP relay service?

Simple Mail Transfer Protocol (SMTP) is a straightforward process that allows users to send, relay, and monitor outgoing emails. An SMTP relay service—also commonly known as an SMTP gateway service—takes this a step further by allowing you to send more transaction and marketing-related emails. Instead of managing the logistics of your own program, you can trust the SMTP relay service to manage and maintain the SMTP server for you. 

But why is it so important to have an SMTP relay service, and how does it work?

Peer-to-peer email hosted via the cloud and accessed on a web browser isn’t sufficient for specific types of mass email campaigns and programs. Many businesses that send out bulk emails, such as newsletters or automated emails triggered by user interaction, need more tailored outgoing email solutions. Utilizing an SMTP relay server for email distribution provides just that.

How does an SMTP relay service work?

When an email service provider (ESP) tries to send an email, it connects to the configured SMTP server and communicates with it using that protocol. Once it can communicate with the configured server, it’s able to deliver the email message through an SMTP relay service. 

SMTP relay works similarly to snail mail—the server essentially acts as the post office. 

Imagine that you have to send someone a physical letter. You write down addresses for both yourself and the recipient on an envelope. After you drop it off at your local post office, the post office then determines which other post office is closest to the recipient and directs the letter there. 

When you send an email, the message includes the recipient’s destination address as well as your return address information—e.g., your email address, IP address, and timestamp. Your local server uses this information to send your message to the recipient’s server for successful delivery

What is an SMTP mail relay?

SMTP mail relay is the process of transferring an email from one server to another for delivery. For example, if you work for Company A and send an email to someone at Company B, you connect to your company’s SMTP server. Then, it delivers your email to Company B’s server. One server accepting an email from another server is “relaying.” 

On the other hand, if you were to send an email to someone with an email address at the same domain as yours, there wouldn’t be a second server involved. This isn’t relaying. It’s like sending a physical letter to someone in your town—your local post office receives and delivers it without involving another post office. 

SMTP mail relay routes emails through a trusted third party to help deliver high volumes of email. It most often specializes in sending large batches of emails, such as newsletters, or automatic transactional emails, such as delivery confirmations and password resets. 

What is a public SMTP relay server?

When an SMTP relay server is public, there’s only so much a business can do before having to search for more costly options. A public SMTP relay server is free to use but becomes limiting once you have to send mass marketing emails. 

Many internet providers put a cap on how many SMTP relays it can conduct per day to combat spam. 

Because of their bulk emailing needs, businesses often exceed the public SMTP relay limit and require enterprise-level email-sending features. An SMTP relay provider can help businesses deliver large amounts of emails without being mislabeled as spam.

Learn more about email infrastructure by downloading Twilio SendGrid’s Email Infrastructure Guide

What is an authenticated SMTP relay? 

SMTP authentication, or SMTP AUTH, is an extension that grants clients access to an SMTP server through an identification mechanism. Authenticated SMTP relay is the process that allows this authentication to happen, verifying the sender’s credentials before processing and relaying the email. This process aims to secure your email delivery, as unwanted parties can’t register with an authenticated SMTP relay server. 

Open mail relays don’t have the same security requirement, and therefore are more prone to being used for the wrong reasons. SMTP authentication helps protect your server from being spammed by malicious emails while allowing you, a trustworthy user, to receive and send emails freely. 

Why is an SMTP relay service important?

Managing your own email server for peer-to-peer email communication is fairly straightforward because most emails only go to a limited number of recipients. But what if you want to send out a marketing newsletter to tens of thousands of subscribers? What if you have a web app that sends out notification emails triggered by your recipients’ activities? Those emails aren’t as unique and often get mistakenly labeled as spam. This is where an SMTP relay service can come in handy. 

If you were to use your primary peer-to-peer email server to take on a large bulk of emailing jobs, your delivery rate would suffer. And you would inevitably clog your bandwidth, potentially delaying the sending and receiving process of peer-to-peer emails.

Constantly monitoring your email server to keep important emails out of the spam folder is an expensive and time-consuming task. 

As your business grows, so do your email-sending needs. 

An outbound SMTP relay service ensures that common delivery mishaps won’t affect your email campaigns—and SMTP relay providers like Twilio SendGrid can help you get started.

Twilio SendGrid’s SMTP relay services

Twilio SendGrid’s SMTP relay services are highly scalable, with no hardware or software costs associated. Our features easily adapt to your business needs and allow you to add additional capacity if needed. Twilio SendGrid also provides a variety of secure SMTP hosting and relay services for transactional email and enterprise jobs. Our transactional email delivery is easy to integrate with multiple developer frameworks and comes with template libraries and code samples

Not only are they easy and cost-effective, but our enterprise email also solutions provide a holistic messaging delivery platform. With Twilio SendGrid’s enterprise email management, you’ll have access to real-time analytics and reporting tools, dedicated account management, and a Support Team to help with any inquiries. 

For more information about SMTP relay services, best practices for emailing, and the ins and outs of email deliverability, check out our article, How to Create a Free SMTP Server (Plus Tips on Buying vs. DIY-ing), or sign up with Twilio SendGrid to set up a free SMTP Service.



With more than 15 years of experience, Carly Brantz is an email deliverability and inbound marketing veteran. As the VP of Revenue Marketing at SendGrid, Carly empowers her team to transform marketing from a cost center to a revenue generator. With a key focus on funnel optimization, Carly leads the team responsible for content creation, lead nurturing, SEO/SEM, and website optimization. An accomplished speaker, Carly shares her email delivery and revenue marketing tactics as a headline speaker on SendGrid’s international roadshow as well as other Marketing conferences.