SMTP relay service: What is SMTP relay & how does it work?

SMTP Relay Service: What Is SMTP Relay and How Does It Work?

Ever wondered how your marketing emails make their way to thousands of inboxes without any issues? Or why some of your messages land in spam while others don't? The answer usually comes down to SMTP relay, the behind-the-scenes system that moves your email from your app to the inbox.

SMTP relay is the backbone of professional email delivery. It routes your messages through SMTP relay servers to reach recipients reliably.

Ultimately, choosing the right SMTP relay service determines whether your emails land in inboxes or get lost in spam folders.

And I think we all know where you'd prefer to end up.

Unlike basic email servers, SMTP relay services:

• Handle high-volume sending
• Manage sender reputation
• Provide the necessary infrastructure for business-critical email delivery

Need a quick refresher on SMTP before we continue? No problem. Watch the short video below:

What is an SMTP relay service?

An SMTP relay service is a third-party email server that accepts your outgoing messages and delivers them to your recipients’ mail servers. Instead of running your own sending server, you point your app or email client at the provider’s server and let it handle delivery.

Simple Mail Transfer Protocol (SMTP) is the standard protocol for sending email. An SMTP relay service, sometimes called an SMTP gateway, uses that protocol to send transactional and marketing email at scale.

Why use one? Everyday email accounts are built for one-to-one messages. Businesses that send newsletters, password resets, or order confirmations to thousands of people need infrastructure built for volume, and that’s what a relay service provides.

How does an SMTP relay server work

When your app or email client sends a message, it connects to your SMTP relay server and hands off the email. The relay server checks your credentials, looks up the recipient’s mail server, and delivers the message there. SMTP relay servers work similarly to snail mail—the server essentially acts as the post office.

Think about mailing a physical letter. You write down addresses for 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 and your return address information—for example your email address, IP address, and time stamp.

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, your company’s SMTP server delivers your email to company B’s server. One server accepting an email from another server is relaying. 

If you send an email to someone with an email address at the same domain as yours, there isn’t 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 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 has limited capabilities when sending mass marketing emails. 

To fight spam, many internet providers cap how many emails you can relay per day.

Because of 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 them getting mislabeled as spam.

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

What is an authenticated SMTP server? 

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 of 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 are more prone to misuse. SMTP authentication helps protect your server from getting spammed by malicious emails while allowing you, a trustworthy user, to receive and send emails securely. 

Which server address and port should you use for SMTP relay?

When you use an SMTP relay service, you point your app or email client at the provider’s server address instead of your own. For Twilio SendGrid, that’s smtp.sendgrid.net.

You’ll also pick a port:

• Port 587: The recommended port for most senders. It upgrades the connection to TLS using STARTTLS.
• Port 465: Uses implicit TLS, so the connection is encrypted from the first byte.
• Port 25: The original SMTP port. Many residential ISPs and cloud providers block it for outgoing mail.

Not sure which to pick? Read our breakdown of port 587 vs. 465. Looking for the server addresses of providers like Gmail or Outlook? See what is an SMTP server.

Why are SMTP relay services important?

Managing your email server for peer-to-peer email communication is relatively straightforward because most emails only go to a limited number of recipients.

But what if you want to send a marketing newsletter to tens of thousands of subscribers? What if you have a web app that sends notification emails prompted by your recipients’ activities?

Those emails aren’t as unique and are often labeled as spam. This is where an SMTP relay service can help. 

If you use your primary peer-to-peer email server to send bulk emails, your delivery rate will suffer. And you may clog your bandwidth, potentially delaying the sending and receiving process of peer-to-peer emails.

Constantly monitoring your email server to keep crucial 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.

Hosted vs. self-managed SMTP relay

You have two ways to run an SMTP relay:

• Self-managed relay: You run mail server software like Postfix on your own hardware or cloud instance. You get full control, but you also own IP warmup, reputation, authentication, blocklist monitoring, security patches, and scaling.
• Hosted relay service: A provider like Twilio SendGrid runs the infrastructure. You connect with a server address, port, and API key, and the provider handles delivery, reputation, and scaling.

Self-managed makes sense for low-volume internal mail or teams with dedicated email engineers. For most businesses sending marketing or transactional email at scale, a hosted relay saves time and headaches. Want to weigh it yourself? Read our guide to creating a free SMTP server.

How to choose an SMTP relay service

The right service depends on how much you send and how much help you want. Compare providers on these factors:

• Sending volume and limits: Check daily and monthly caps, and how pricing scales as you grow.
• Deliverability tools: Look for dedicated IP options, IP warmup, and reputation monitoring.
• Authentication support: SPF, DKIM, and DMARC setup should be simple. Gmail, Yahoo, and Microsoft now require authentication from bulk senders.
• Security: TLS encryption, API key authentication, and access controls protect your account from abuse.
• Analytics: Real-time data on deliveries, bounces, and spam complaints helps you catch problems early.
• Support and uptime: Check support hours, response times, and published uptime commitments.
• Developer experience: Clear docs, code samples, and both SMTP and API options make integration faster.

How to troubleshoot SMTP relay service issues

An SMTP relay service simplifies email delivery, but like any technology, it can run into problems. Here are some troubleshooting tips for the most common issues such as IP blocklisting, high bounce rates, and delays in email delivery.

Dealing with IP blocklisting

IP blocklisting occurs when your sending IP address is added to a blocklist used by email providers to block spam. This can severely affect your email deliverability. Here's how to fix it:

1. Monitor your IP reputation regularly: Use tools like SenderScore, Talos Intelligence, or MXToolbox to check your IP reputation. These services can alert you if your IP gets blocklisted.
2. Warm up your IP: If you’re using a new IP address, start by sending a small volume of emails and gradually increase the amount. This helps to build a positive sending reputation with ISPs.
3. Follow email sending best practices: Ensure your email lists are clean and up-to-date, avoid spammy content, and use double opt-in methods for subscriber lists to maintain list quality.
4. Request de-listing: If you find your IP address has been blocklisted, contact the blocklist operator and request removal. Be prepared to explain how you’ve remedied the issue that caused the listing.

Handling high bounce rates

Bounce rates can spike due to invalid email addresses, server issues, or aggressive filtering by ISPs. Here's how to fix it:

1. Validate email addresses: Use email validation services before adding addresses to your mailing list to ensure they are correct and active.
2. Segment your lists: Regularly clean your email list by removing inactive subscribers who do not engage with your emails over a significant period.
3. Review email content and frequency: Ensure your content is relevant and not too frequent to avoid recipient fatigue. Check for triggers that might cause spam filters to block your emails.
4. Analyze bounce messages: Pay attention to the bounce messages you receive as they can provide clues on why emails are not being delivered.

Resolving delays in email delivery

Delays can occur due to network issues, server overload, or configuration errors. Here's how to fix it:

1. Check server load: Ensure your SMTP server is not overloaded with requests. If persistent delays occur, consider scaling up your server resources.
2. Optimize email size: Large emails with high-resolution images or attachments can slow down delivery. Optimize these elements to speed up the process.
3. Review configuration settings: Incorrect settings in your SMTP server can cause delays. Verify your configuration settings are correct for timeouts, retry intervals, and maximum message sizes.
4. Contact your SMTP service provider: If issues persist, contact your SMTP relay service provider. They can provide insights into potential network issues or configuration errors that are not immediately obvious.

Avoid common delivery mishaps with Twilio SendGrid’s SMTP services

Twilio SendGrid’s SMTP relay services are highly scalable, with no hardware or software costs associated. Our features adapt to your business needs and allow you additional capacity if needed. 

Twilio SendGrid also provides various secure SMTP hosting and relay services for transactional email and enterprise jobs. Our transactional email delivery integrates with multiple developer frameworks and comes with template libraries and code samples. 

Not only are these efficient and cost-effective, but our enterprise email solutions also provide a holistic messaging delivery platform.

With 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 (Buying vs. DIYing), or sign up with Twilio SendGrid to set up a free account.

Frequently asked questions about SMTP mail relay

What is an SMTP relay service?

An SMTP relay service is a third-party email server that sends your messages for you. You route outgoing email through the provider’s servers, which handle delivery, reputation, and bounces so your email reaches the inbox reliably and securely.

How does an SMTP relay service improve email deliverability?

SMTP relay services help improve deliverability by managing the reputation of the sending IP addresses, implementing proper authentication protocols (like SPF, DKIM, and DMARC), and handling bounces and feedback loops with ISPs.

What are the differences between a public and a private SMTP relay server?

A public SMTP relay server is typically free to use but may have limitations in terms of sending volume and is generally less secure. A private SMTP relay server is a paid service that offers greater control, higher sending limits, enhanced security, and better deliverability.

Is SMTP relay the same as SMTP authentication?

No, they are different. SMTP relay refers to the process of transmitting emails from one server to another. SMTP authentication, on the other hand, is a protocol used to authenticate the credentials (like username and password) of the sender to prevent unauthorized usage of the SMTP server.

How do I choose the right SMTP relay service?

Choosing the right SMTP relay service depends on several factors, including your email volume, budget, specific features like analytics and reporting tools, the level of customer support provided, and compliance with security standards.

These features separate enterprise-grade solutions from basic offerings:

• Dedicated IP addresses: Protect your sender reputation with dedicated SMTP relay servers.
• Authentication protocols: Built-in SPF, DKIM, and DMARC configuration.
• Scalable infrastructure: Handle volume spikes without performance degradation.
• Real-time analytics: Monitor mail relay performance and deliverability metrics.