What is SMTP relay and why you need it to send bulk emails

If you're planning to send a high volume of transactional emails, you'll want to ensure your emails servers are set up for success. The last thing you want is to end up in the spam or junk folder. You're in luck! You have a few options for sending bulk emails, including:

1. Connecting using an email API, which requires programming, installing libraries, and keeping to date with changelogs.

2. Building your own dedicated SMTP relay server, which requires technical expertise to set up and maintain.

3. Using a third-party SMTP relay service, like MailerSend, that does all the backend work for you so you can focus on results.

The first two options are great for organizations that have teams of developers and big budgets. But for the rest of us, third-party SMTP relay services level the playing field and make your life easy by handling all the technical aspects of the delivery.

What is an SMTP relay service?

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

SMTP stands for Simple Mail Transfer Protocol, a communication protocol for electronic messages that was established in 1982. As such, it’s the industry standard used by mail servers to send, receive and relay emails over the Internet.

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.

SMTP relay services like MailerSend are typically used for sending transactional emails like account opening emails, one-time passwords and shipping notifications.

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.

How does an SMTP relay work?

We take it for granted that an email is sent and delivered after clicking on “send”. In reality, your email 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). It checks your email for correctness, including missing “To” fields, before passing it to a Mail Transfer Agent (MTA). 

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.

In the cloud, the MTA checks the Mail Exchange (MX) records of the recipient’s domain. This record tells the email to which server it should go. Your email follows the route and is transferred to another MTA before it is accepted by a Mail Delivery Agent (MDA).

The MDA receives your email, converts it to a readable format and then passes it to the recipient’s MUA. After taking a complex journey lasting seconds, your email then finally appears in the inbox of the recipient’s email app!

Why you need a third-party SMTP relay service

So, you’re excited to integrate transactional emails like welcome emails, password resets and order confirmations into your workflow. Here’s how an SMTP relay service will benefit your deliverability and performance in the long run.

Scale your email sendings

The flexibility to scale your transactional emails is the number one reason why you need a third-party SMTP provider to send mass emails on your behalf. 

Popular web hosts and webmail providers like GoDaddy and Outlook.com 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. How would you feel waiting until the next day for a password reset email?

A hosted SMTP relay service solves these sending limitations by providing a generous sending quota every month. Also, you can control your costs of scaling by paying as you go or by subscribing to a monthly plan if you’re a high-volume sender.

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. 

Authenticate your emails

How do popular ISPs like Gmail, Yahoo and Outlook.com know that a message is really from who it appears to come from? 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.

Thankfully, 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 or junk 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.

Authentication records in MailerSend

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. You would rather focus on getting your message out than deal with these technical issues!

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.

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.

Sending emails using MailerSend

SMTP is probably the most popular way of sending outgoing emails for most people. There’s no coding involved and you only need to plug in 4 pieces of information into your website, app or device! Here’s how to get started sending emails with MailerSend.

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 click on the switch next to Send emails using our SMTP servers instead of our REST API to enable SMTP.

SMTP section in MailerSend

Turning on the switch will automatically generate the following four pieces of information:

  • Server (SMTP host)

  • Port

  • Username

  • Password

SMTP details in MailerSend

Note: Here’s a quick way to copy the contents to your clipboard. Simply hover over the fields and click on them. For the password, click on the eye icon to reveal the password and then click on it.

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…

Thunderbird setup screen

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

Thunderbird SMTP details screen

Enter the following information from MailerSend:

  • Server: smtp.mailersend.net

  • 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 the popular WP Mail SMTP plugin.

Note: Why not just use the built-in email feature in WordPress? WordPress uses the PHP mail function to send emails. However, it’s not reliable and your emails are often marked as spam. Furthermore, most hosting companies don’t configure PHP mail properly, or worse, simply disable it to prevent spam.

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.

WP Mail SMTP settings

Enter your SMTP details from MailerSend:

  • SMTP Host: smtp.mailersend.net

  • 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.

Essential SMTP relay glossary

Domain Name System (DNS)

The Domain Name System is a global directory of domain names matched with their underlying IP address where the website is stored.

Email relay

Like a relay race, an email relay describes the process of handing an email from one server to another until it reaches the finish line—the recipient’s inbox.

Internet Service Provider (ISP)

An ISP is a general term for businesses that provide services related to the Internet. Some examples include high-speed broadband services (Verizon), mailbox services (Outlook.com) and web hosting services (GoDaddy).

Mail Delivery Agent (MDA)

Also known as a Message Delivery Agent, this is the software responsible for the last mile delivery of email to the recipient’s inbox.

Mail Exchanger (MX) Record

An MX record is part of the DNS that specifies which mail server will accept emails on behalf of a domain name. 

Mail Submission Agent (MSA)

Also known as a Message Submission Agent, it describes the software that receives email from a Mail User Agent (MUA) and works with a Mail Transport Agent (MTA) to deliver the email.

Mail Transport Agent (MTA)

This software, also known as a Message Transport Agent, transfers email from one server to another using SMTP.

Mail User Agent (MUA)

A Mail User Agent is an email client that is used to receive, send and manage email messages.

Simple Mail Transport Protocol (SMTP)

SMTP is a communication protocol for electronic mail transmission established in 1982. Email servers and MTAs use SMTP to send and receive email messages.

SMTP relay

An SMTP relay is an email service that routes high volumes of email messages through SMTP servers.

Secure Sockets Layer (SSL)

SSL is a cryptographic protocol used to secure communication between computers on a network. It was replaced by TLS in 1999.


STARTTLS is a command sent from an email client to an email server. It informs the server to upgrade a previously insecure connection into a secure one.


Transport Layer Security is a newer cryptographic protocol that evolved from SSL. Because of this, TLS and SSL are sometimes used interchangeably.

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!

I’m Sean, Content Writer at The Remote Company. Like many others, I've rediscovered the joy of cycling. My next goal is to build a bike from scratch using mail-order parts. Wish me luck.