What’s an SMTP Relay Service

What’s an SMTP Relay Service
Denys Romanov Denys Romanov 13 february 2024, 12:17 477
For beginners

Sending bulk emails to your subscribers or customers may seem easy. You write something clever in a good email design and press "Send", then sit back and wait for clicks and purchases. Unfortunately, you might also encounter problems ranging from poor email deliverability to blacklisting. Yet most of them can be resolved using an SMTP relay service.

SMTP stands for "Simple Mail Transfer Protocol." This protocol is usually implemented as an SMTP server, a complex software package. This backend system helps you and your company transfer emails between you and your recipients.

Maintaining your own SMTP server requires much skill and effort, especially within email marketing demands. Luckily, email service providers (ESPs) offer ready-to-use SMTP servers as a service. Let's find out what an SMTP relay service is and how it can benefit your business.

What is an SMTP Relay Service?

The Simple Mail Transfer Protocol is a protocol for sending, relaying, and monitoring outgoing emails. An SMTP relay service, also known as an SMTP gateway service, extends this capability, allowing you to send large volumes of transactional and marketing-related emails while maintaining high speed and deliverability.

For some bulk email campaigns and programs, using just your own SMTP server may be insufficient. Businesses like yours that send out bulk emails, newsletters, or automated emails triggered by user interaction, need customized outgoing email solutions. Using an SMTP relay server for email distribution is a way to accomplish this.

How Does an SMTP Relay Service Work?

SMTP relay is similar to a traditional postal service, with the server being the post office. When sending a letter on paper, you write down your own and your recipient's addresses. After handing it over to your local post office, the post office determines which other post office is nearest to the recipient's address, and sends the letter there.

SMTP mail relaying is the process of moving emails from one server to another for delivery. If you work for Company A and want to send an email to someone at Company B, your email client software connects to the SMTP server at your company, and it sends your email to the server of Company B.

With SMTP relay, emails are routed through a trusted third party server capable of handling high volumes of email. This solution is most commonly used to send large batches of emails, such as newsletters, transactional emails, delivery confirmations, and password resets.

Why Do You Need an SMTP Relay Service?

When your emails only go to a small number of recipients, running your email server is fairly simple. But what if you are sending a marketing newsletter to tens of thousands of people, or you have a web-based system that sends out notification emails based on the recipient's actions? These emails are frequently mislabeled as spam due to the high sending volumes.

If you use your primary email server to handle bulk emailing, your delivery rate may suffer. You'd also clog up your connection bandwidth, potentially slowing down the sending and receiving of other important emails.

Also, keeping your email server up and running 24/7 requires additional resources in terms of dedicated stuff and money.

As your company grows, so will your email sending requirements. SMTP relay providers like UniOne come to the rescue and help you get started with an outbound SMTP relay service, guaranteeing that common delivery mishaps won't affect your email campaigns.

How to Start Sending Email Through an SMTP Server?

At UniOne, a single SMTP connection allows sending up to 5000 emails per hour, and with 10 simultaneous connections, you can send up to 50 000 emails per hour. Integrating the UniOne SMTP is extremely easy and can be done within minutes; you only have to set a few connection parameters. Check out our SMTP API documentation for more information.

You can make use of UniOne's advanced features like substitutions (merge tags), templates, links, and read tracking when sending via SMTP. To use these features, you add a custom X-UNIONE message header, which contains parameters in JSON format.

If speed and versatility is critical for your business, consider using our Web API: it allows you to send millions of emails per hour and offers numerous additional benefits.


SMTP relay simplifies your life as an email marketer and business owner by taking care of all the back-end heavy lifting, allowing you to spend more time crafting perfect content for your customers and expanding your contact lists. UniOne's SMTP relay services are highly scalable and come with no added hardware or software costs. Our features adapt quickly to your business needs and allow you to scale up as needed. Transactional email delivery comes with template libraries and code samples and is easy to integrate with diverse developer frameworks.

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