What is SMTP relay and how does it help email deliverability

Amy Elliott Amy Elliott
· 10 min read · Tips and resources · December 27th, 2022
Want to learn more about SMTP? You’ve come to the right place! Here’s everything you need to know about SMTP relay so you can quickly integrate email sending into your workflow.

In transactional email, there’s a lot going on behind the scenes to ensure your emails get sent and recipients receive your most important messages in a timely manner. SMTP is one protocol that facilitates this.

SMTP (Simple Mail Transfer Protocol) is a protocol for sending email messages between mail servers. Most email systems that send mail over the internet use SMTP to send messages from one server to another, and to deliver messages to local mail clients like Gmail, Microsoft Outlook or Apple Mail. 

But what are SMTP relays and how do they fit in? In this guide, we’ll break down how SMTP relay servers ensure your messages get from A to B (and all the places in between!). Ready? Let’s go!

What is an SMTP relay and how does it work?

An SMTP relay is an email relay service that routes high volumes of email messages through SMTP servers so they don’t end up in the spam folder. 

A relay, or mail relay, describes the zig-zag process of passing an email from one email server to another until it reaches the recipient.

New to transactional emails? Learn more about the different types of transactional emails and how to use them in your email strategy with the Ultimate guide to transactional emails.

When an email is sent, it needs to follow a complicated route and jump through a couple of hoops before it magically lands in your recipient’s inbox.

SMTP relay flow

When you click “send” in your email app or Mail User Agent (MUA), your email is sent to your server’s Mail Submission Agent (MSA). Think of the MSA as your mail carrier. It checks your email for correctness, including missing “To” fields, before passing it to a Mail Transfer Agent (MTA) or, your local post office. 

Note: If your recipient happens to use the same @domain or resides on the same server, your email can be delivered straight to them without using an SMTP relay.

The MTA (sender’s local post office) checks the Mail Exchange (MX) records of the recipient’s domain. This tells the email which server it should go to, kind of like an address! Your email follows this route and is transferred to another MTA (receiver’s local post office) before it is accepted by a Mail Delivery Agent (MDA), or the delivering mail carrier.

The MDA (mail carrier) receives your email, converts it to a readable format and then passes it to the recipient’s MUA. After this complex journey (which lasts only seconds!), your email then finally appears in the inbox of the recipient’s email app!

OK, so what’s an SMTP relay service?

Implementing SMTP and executing the mail relay process requires a complex sending infrastructure. Rather than try to do this themselves, businesses can choose an SMTP relay service to do all the hard work for them. 

SMTP relay services or email service providers deliver the sender’s email messages by routing them via a trusted third party, ensuring they reach their destination in a safe, secure and reliable way. SMTP services like MailerSend are typically used for sending transactional emails, for example, welcome emails, one-time passwords and shipping notifications.

Try MailerSend’s SMTP relay service

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When to use an SMTP relay service

An SMTP relay service takes all the hassle out of transactional email and improves your deliverability. You should think about using a third-party SMTP relay service if you want to:

1. Scale your email sendings

Popular web hosts and webmail providers like GoDaddy and routinely impose sending limits on their SMTP servers. These limits range from 300 to 500 emails per day, with some putting a further restriction of 100 recipients per email!

While these restrictions are meant to discourage spammers, they can have a detrimental impact on your customer experience if your email takes several hours to be delivered. 

Using a hosted SMTP relay service to send mass emails on your behalf solves these sending limitations by providing a generous sending quota every month. Also, you can control the cost of scaling by paying as you go or by subscribing to a monthly plan if you’re a high-volume sender.

2. Protect your sending reputation

Behind every domain name is an IP address from which emails are sent. Web hosting companies often share one IP address among several hundred websites on a single server. That means you are likely sharing an IP address with other domains!

If you are sharing an IP address, your sending score is affected by the actions of other websites on the server. There are two ways you can regain control: by using a dedicated IP address or by using a third-party SMTP relay service.

A trusted SMTP relay provider protects your sending reputation and thus avoids email deliverability issues. The use of clean IP addresses ensures that you stay off email blocklists, while comprehensive filters make sure that no spam leaves your IP address. 

3. Authenticate your emails

Email spoofing is used in spam and phishing attacks where an email header is changed to make it look like it was sent from a trusted sender. So how do popular Internet Service Providers (ISPs) like Gmail, Yahoo and know that the sender of an email is authentic?

Email security protocols like SPF, DKIM and DMARC are used to authenticate your emails. These records prove that you’re legitimate, that your identity hasn’t been compromised, and that you’re not sending emails for someone else.

Adding these protocols to your DNS records, however, is tricky if you want to ensure that your emails reach the inbox and not the spam folder. Setup instructions and support can vary a lot between SMTP servers from webmail and hosting providers.

An SMTP relay provider like MailerSend automatically generates optimized authentication records for you. A custom return-path is even provided to receive bounced emails and to further protect your sending reputation.

4. Secure your email

SMTP relay can be secure, but it also depends on how it is configured and used. By default, SMTP uses plain text to transmit messages, which means that the messages can be intercepted and read by anyone with access to the network. 

To protect the confidentiality of the message, it’s important to use an encryption method like Transport Layer Security (TLS) or Secure Sockets Layer (SSL) to secure the connections between the client and the server.

What’s more, it’s crucial to properly configure the SMTP relay to prevent unauthorized use. This could include setting up authentication methods to send messages through the relay and limiting the IP addresses or domains that are allowed to use the relay. 

A third-party service makes securing your email easy by implementing the necessary security protocols and providing additional security features such as 2FA and IP allowlisting.

5. Comply with privacy regulations

A third-party SMTP service can help senders comply with privacy regulations by providing the necessary security features along with privacy controls and policies. This can be particularly useful when sending sensitive or confidential information, such as personal data or financial information. It can offer:

  • Compliance with privacy regulations such as the EU’s General Data Protection Regulation (GDPR) or the California Consumer Privacy Act (CCPA). Third-party services like MailerSend often provide features for managing consent or opt-out preferences, or for implementing data protection controls.

  • Secure servers which are used to store and transmit emails, helping to protect against unauthorized access or data breaches.

  • Access controls, as mentioned above, that allow users to set permissions so that only authorized individuals can access sensitive information.

6. Save time and effort

Installing your own email server is hard work. Complex settings need to be configured and you need to ensure 24/7 uptime and secure your systems against malicious hackers. 

Leave it to email experts to manage a hosted SMTP relay service. Dedicated technical teams will take care of optimizing the server for performance and deliverability, while you can get support when you need it most from a 24/7 customer support team.

7. Monitor and optimize for performance

When you send emails through your regular ISP, how do you assess the performance of your outbound emails? Other than bounced email replies, basic reporting and analytics may be lacking with SMTP servers from webmail providers and web hosts.

In order to maintain a good customer experience and identify sending issues, you will want to know email sending metrics like:

  • Was your email delivered to the recipient?

  • Why did your emails bounce?

  • How many recipients opened their emails and how many clicked through?

  • Did anyone unsubscribe or mark your email as spam?

An SMTP relay provider like MailerSend can provide you with a real-time feed of your recipient activity. Troubleshoot sendings by filtering results and monitor email metrics like opens, clicks and unsubscribes to improve engagement and email delivery rates.

How to get started with MailerSend’s SMTP relay service

Setting up MailerSend for your outgoing emails is easy; you only need to plug in 4 pieces of information to your website, app or device! Here’s how to get started: 

Check out our online guide to SMTP relay any time you need help.

Retrieve your SMTP details

Navigate to the Domains page and find the domain name that you wish to send SMTP mail from. Click on Manage next to the domain name.

Domains page in MailerSend

Scroll down to the SMTP section and toggle Send emails using our SMTP servers instead of our REST API on to enable SMTP.

Enabling this will automatically generate the following four pieces of information:

  • Server (SMTP host)

  • Port

  • Username

  • Password

Note: MailerSend’s SMTP relay only uses port 587, as this is the only way to properly secure SMTP connections. Learn more about SMTP ports.

If you need to select encryption in your app, select Transport Layer Security (TLS) or STARTTLS. This is a newer and more robust protocol than Secure Sockets Layer (SSL) for secure email sending.

Set up SMTP email for your email client or app

We’ll use Thunderbird, a free and open-source email client, as an example here. Go to File, New, and then select Existing Mail Account

Enter your name and email address, and click on Configure manually…. This will show additional SMTP fields for outgoing emails.

Enter the following information from MailerSend:

  • Server:

  • Port: 587


  • SMTP username and password

Test your SMTP configuration by clicking on Re-test. Once verified, click Done to complete the setup. You can now send emails through an SMTP relay service!

Set up SMTP email for WooCommerce and WordPress

Your WooCommerce store depends on WordPress to send emails. WordPress itself uses SMTP plugins to send emails via an SMTP server. We’ll show you how it’s done in WordPress with MailerSend’s SMTP plugin as well as the popular WP Mail SMTP plugin.

Note: WordPress has an inbuilt email feature that uses the PHP mail function to send emails. However, it’s not reliable and emails are often marked as spam. Furthermore, most hosting companies don’t configure PHP mail properly or simply disable it.

MailerSend’s SMTP plugin

Find MailerSend - Official SMTP Integration in the WordPress Plugin Directory or upload the plugin to your server. Once installed, activate it and then go to the plugin in the WordPress admin. You’ll see the following page:

MailerSend's SMTP plugin admin page in WordPress.

Enter your SMTP username and password from MailerSend. You can also fill in the Sender details and send a test email. Click Save and you’re all done!

WP Mail SMTP plugin

Find WP Mail SMTP in the WordPress Plugin Directory or upload the plugin to your server. Once installed, activate it and then go to WP Mail SMTP in the WordPress admin. Choose Other SMTP and you’ll see the following page.

Enter your SMTP details from MailerSend:

  • SMTP Host:

  • Type of Encryption: STARTTLS

  • SMTP Port: 587

  • SMTP Authentication: Yes

  • SMTP username and password

Note: If you’re using another SMTP plugin and you don’t see STARTTLS for encryption, you can choose the TLS encryption instead.

Click Save Changes when done.

Congratulations! You have just improved the sending reliability of your WordPress emails by using a cloud-based SMTP email server.

Unleash your email sendings with SMTP relay

With a third-party SMTP relay service, you can send large volumes of email without the hassle of coding or setting up a server. Pick a trusted email provider (hint: MailerSend) who can help you land in the inbox, protect your sending reputation, and improve your email performance.

Have you ever set up an SMTP server on your own? How did it go? Share in the comments!

Ready to elevate your transactional email?

Try MailerSend’s SMTP relay service for free. Start with 3,000 emails/month for free then pay as you go!

Editor’s note: This post was originally published in February 2021. We've now updated it with additional information.

Amy Elliott
I’m Amy, Content Writer at MailerSend. As a child, I dreamt about writing a book and practiced by tearing pages from an A4 notepad and binding them with sugar paper. The book is pending but in the meantime, I love taking a deep dive into technical topics and sharing insights on email metrics and deliverability.