An SMTP Relay keeps your emails on track. It ensures they arrive fast and secure.
This guide covers how it works, why businesses rely on it, and how to choose the best provider. You'll learn best practices for smooth delivery, troubleshooting tips, and insights into top providers like SendGrid, Mailgun, Brevo, and Postmark.
Without further ado, let’s begin.
What is SMTP Relay and why Does it Matter?
SMTP (Simple Mail Transfer Protocol) relay is a process (service or server) that helps move emails across different mail servers. When using it, the email passes through a third-party server first instead of going straight from your email to the recipient. This extra step helps in many ways, such as improving security and delivery rates and keeping emails from getting marked as spam.
The SMTP relay works for both transactional emails (like password resets) and bulk messages (like newsletters). So, marketers or app developers can use it to send large volumes of emails.
SMTP Relay: Why It Matters
In summary, SMTP relay makes sure your emails don’t get stuck. It ensures emails get where they need to go, without delays or security risks. It is like a postal service for email. Furthermore, it moves messages smoothly and securely from the sender to the recipient—even across different domains.
No matter the type of email (transactional emails, bulk emails, or marketing emails), SMTP relay will make sure your emails land in the right inbox—fast, secure, and spam-free.
Why Businesses Rely on SMTP Relay?
- Better Deliverability Emails go through trusted servers, reducing the risk of landing in spam.
- Scalability Send thousands or even millions of emails without hitting ISP limits.
- Stronger Security Authentication tools like SPF, DKIM, and DMARC block spoofing and phishing.
- Protects Your Reputation Keeps your sender score high by following best practices and managing IPs.
- Detailed Analytics Get insights on delivery rates, bounces, and opens to improve campaigns.
How does SMTP Relay Work?
SMTP relay moves emails through a relay server before they reach the recipient. Refer to the image below.
- Sender Initiates Email:
You or your application creates and sends an email, whether a transactional message (e.g., password reset) or bulk email (e.g., newsletter). - Outgoing Mail:
The email is sent from the sender’s email client (e.g., Gmail, Outlook) or application to an SMTP Relay server, which routes and authenticates it. - SMTP Relay Server:
The server handles: - Authentication: Verifies sender credentials.
- Encryption: Secures the email using TLS.
- Routing: Determines the best delivery path.
- Recipient’s Mail Server:
The email is forwarded, and the recipient’s server checks domain records (SPF, DKIM, DMARC) for legitimacy. - Email Delivery:
If verified, the email reaches the inbox. Otherwise, it may be rejected or flagged as spam.
Note: Software companies like SendGrid or Mailgun offer SMTP relay services to easily handle large-scale email sending. More on these software companies later on.
How to Choose the Right SMTP Relay Provider
To start with an SMTP Relay Service, you must first choose a reliable provider. Select a service like SendGrid, Mailgun, or Gmail SMTP relay based on your needs.
How to start?
- Create an Account Sign up and get your SMTP credentials.
- Verify Your Domain Set up SPF, DKIM, and DMARC records for authentication.
- Configure SMTP Settings Enter the SMTP server, port, and login details in your email client or app.
- Test the Setup Send a test email to ensure everything works.
Consider these factors and why they matter when selecting a provider:
| Factor | Description | Why It Matters |
|---|---|---|
| Email Volume | Supports sending needs from thousands to millions of emails | Ensures scalability without hitting ISP limits |
| Security | Offers TLS encryption, SPF, DKIM, and DMARC support | Protects against spoofing, phishing, and unauthorized access |
| Deliverability Tools | Provides analytics and reputation tracking | It helps prevent emails from landing in spam folders |
| Integration | Some providers offer API access for advanced use | Allows seamless connection with existing systems and apps |
| Pricing & Scalability | Plans fit budgets and grow with business needs | Ensures cost-effectiveness while handling increasing demand |
Top SMTP Relay Providers
1. SendGrid
SendGrid (acquired by Twilio in 2018) is a cloud-based email delivery service. It delivers 134 billion emails monthly with speed, security, and reliability. Its strong infrastructure, dedicated IPs, and authentication tools aim to keep emails out of spam. SendGrid's SMTP Relay Service allows you to send emails reliably via SMTP or its RESTful API. It supports enhanced email sending with its SMTP API.
Key Features:
- High Deliverability: Avoids spam filters and reduces bounces.
- Domain Authentication: Verifies sender identity for trust.
- Reputation Management: Protects sender score with insights.
- Email API: Enables reliable sending, tracking, and validation.
- Automation & Analytics: Streamlines campaigns and engagement.
Why do we recommend it?
We recommend SendGrid because it is reliable, user-friendly, and great for delivering transactional emails. The API is well-documented, and the no-code builder integrates smoothly with other tools.
Who is it recommended for?
SendGrid is perfect for developers needing a simple API for email. It is also ideal for marketing teams wanting automation and tracking. Small and mid-sized businesses like it, because of its scalable and budget-friendly setup.
2. Mailgun
Mailgun is a cloud-based email delivery service designed for developers to send, receive, and track transactional emails. Founded back in 2010 by Ev Kontsevoy and Taylor Wakefield, this email API service has now become a trusted platform for developers. Mailgun provides a Free SMTP Relay Service for transactional and application emails. But aside from SMTP server and SMTP relay it also features real-time tracking, advanced analytics, detailed logs, email authentication, and more.
Key Features:
- Email API: Seamless sending, receiving, and tracking.
- Email Validation: Real-time address verification to reduce bounces.
- Deliverability Tools: Optimize inbox placement and reputation.
- Inbox Testing: Check email placement and improve delivery.
- Spam Trap Monitoring: Prevent reputation damage from spam traps.
Why do we recommend it?
We recommend Mailgun because it makes email delivery simple, fast, and reliable. But what’s special about this tool is its developer-friendly API and advanced tools which ensure emails reach inboxes, not spam folders.
Who is it recommended for?
Mailgun is ideal for developers working in businesses that send a lot of emails. We also recommend it to startups for its flexibility, all the way to large companies for its power to streamline email delivery.
3. Brevo (formerly SendinBlue)
Brevo (former SendinBlue) was founded in 2012 and has since evolved to provide a wide range of digital marketing solutions. Today it offers a complete suite of tools for businesses to manage their email marketing, SMS campaigns, chat, and customer relationships. Brevo's SMTP relay service is quite popular for how reliable it is in sending transactional emails. It also ensures minimal latency through geographically distributed relays. These relays automatically connect users to the best-performing server.
Key Features:
- Email Marketing: Create and track campaigns with templates and analytics.
- Transactional Emails: Securely send critical messages via SMTP relay.
- Marketing Automation: Automate engagement with behavior-based workflows.
- Landing Page Builder: Design and launch pages without coding.
Why do we recommend it?
We recommend Brevo because it is a budget-friendly, powerful marketing tool. It’s easy to use yet packed with advanced tools like automation and transactional emails. Unlike most competitors, Brevo charges per email sent, not per subscriber. This makes it a smart choice for businesses with large contact lists.
Who is it recommended for?
Brevo is perfect for small and mid-sized businesses (e-commerce, stores, startups, etc.). It’s great for those businesses that need automation, segmentation, and transactional messaging without spending too much.
4. Postmark
Postmark is a cloud-based email (SMTP and API) delivery service. They specialize in the fast and reliable delivery of transactional emails. Postmark (founded in 2009) has established itself as a top-tier SMTP email service provider. Thanks to Postmark’s built-in infrastructure, the SMTP Relay service can provide a reliable sending environment. It removes the hassle of managing servers.
Key Features:
- High Deliverability: Avoids spam with strong anti-spam measures.
- Secure SMTP: Ensures safe delivery with TLS encryption.
- Easy Integration: Seamlessly connects with platforms like WordPress.
- Strong Security: Protects data with advanced encryption.
Why do we recommend it?
With Postmark deliverability features your messages won’t get lost in spam or mixed with marketing blasts. We recommend this tool because the setup is simple with either SMTP or API integration. Security is also a plus. It is solid with TLS encryption and DMARC monitoring.
Who is it recommended for?
Postmark is ideal for those (especially startups or small businesses) looking to send transactional emails. E-commerce stores can also use it for its fast order and shipping updates. We also recommend it for developers and SaaS teams for the smooth API and SMTP setup.
Other SMTP Relay for Popular Email Services
a. Gmail SMTP Relay
Gmail SMTP Relay allows you to route outgoing emails through Google's servers instead of their own SMTP. This is useful if your business uses Microsoft Exchange or other SMTP servers. So, what are the benefits? Overall, you’ll get better email deliverability. You can enforce security policies and filter spam and viruses.
How to set up?
- In Google Admin Console
- Go to Apps > Google Workspace > Gmail > Routing.
- Configure Allowed Senders, Authentication, and TLS Encryption.
- Save changes (may take up to 24 hours to apply).
- On your mail server
- As per documentation, point it to Google’s SMTP servers > Configure Exchange, Postfix, Sendmail, etc.
b. Office 365 SMTP Relay
Office 365 SMTP Relay allows you to send emails through Microsoft 365 without requiring individual mailboxes. This relay solution is ideal for printers, scanners, apps, and automated notifications. The benefits? It supports external recipients, uses IP-based authentication via a connector (so no mailbox authentication is needed), and has high sending limits.
How to Set Up?
- Create Connector: Admin Center > Exchange > Mail Flow > Connectors
- From: Your email server → To: Microsoft 365
- Auth: Static IP or TLS certificate
- Configure SMTP on Devices/Apps:
- Server: Your MX endpoint
- Port: 25 | TLS: Enabled | Auth: None
SMTP Relay: Best Practices & Fixes
You can keep your SMTP relay running smoothly with the right setup and a little know-how,
Let’s break it down into simple steps and fixes.
Best Practices for SMTP Relay Use
First things first: pick a reliable SMTP relay provider. A good provider keeps your emails moving without roadblocks. Once you have that sorted, lock down your email security with SPF, DKIM, and DMARC. These authentication protocols stop spammers from pretending to be you and help your emails look legit to inboxes.
Next, think about who you’re emailing. It is a bad idea to send the same message to everyone. Instead, split your list into smaller groups “segments” and send targeted emails. This keeps people interested and cuts down on hard or soft bounces. It is also a good idea to clean up your list regularly. Too many bad “invalid” addresses can hurt your reputation.
Quick Tips:
- Use a trusted SMTP provider.
- Set up SPF, DKIM, and DMARC.
- Segment your email list.
- Clean your list to avoid bounces.
- Test your setup often.
Troubleshooting SMTP Relay Issues
If your emails are not sending, don't panic. Start with the basics: check your username, password, and API key. A typo here can stop everything. Next, peek at your domain settings. If SPF, DKIM, or DMARC records are wrong, your emails might get flagged as spam.
SMTP logs are quite useful when things go wrong. They’ll tell you exactly what’s broken. Also, double-check your server settings. A wrong address or port will stop emails cold. Also, don't forget to keep an eye on your IP reputation. If it gets blacklisted, your emails won’t go far. A dedicated IP or separate domain can help avoid this.
Quick Fixes:
- Check your login details.
- Fix SPF, DKIM, and DMARC records.
- Look at SMTP logs for errors.
- Confirm server address and port.
- Monitor IP reputation.
Conclusion
SMTP Relay keeps your emails on track. It takes email deliverability to the next level. It makes sure they land in inboxes—not spam folders.
Without an SMTP relay, your messages might get lost in spam folders, blocked by ISPs, or even faked by scammers.
Luckily, services like SendGrid, Mailgun, Brevo, and Postmark handle these headaches for you. They make sure your emails are delivered, verified, and trusted—so you don’t have to worry about authentication or IP reputation.
Best part? Each platform offers free plans, so you can test them out without spending a dime. We recommend them for you if you're a startup, a fast-growing business, or a developer sending thousands of emails.
Try these SMTP services today. And get fast and spam-free email delivery!
SMTP Relay FAQs
How does SMTP relay affect email deliverability?
SMTP relay helps manage large email volumes, but a poor setup will definitely hurt deliverability. Spam filters will block emails from unknown or misconfigured servers. A good SMTP relay service, on the other hand, improves email flow and inbox placement.
SMTP Relay vs. Email API—What’s the Difference?
SMTP Relay is a no-code solution for basic email delivery. Email APIs on the other hand offer faster, customizable email sending but require coding. Use SMTP for simplicity or APIs for advanced features and scalability.
How is SMTP Relay Different from an SMTP Server?
An SMTP server sends and receives emails directly. An SMTP relay is the middleman, passing emails along. Relay is for bulk sending.
How Do SPF, DKIM, and DMARC Prevent Spam?
These are email authentication protocols. They tell mail servers your emails are legit. SPF verifies senders, DKIM secures email content, and DMARC blocks fakes. Without them, your emails are more likely to get flagged as spam.
How Can I Avoid SMTP Relay Blacklisting?
Avoid SMTP Relay blacklisting by using authentication (SPF, DKIM, DMARC). Ideally, you should maintain a good sender reputation by sending quality content, managing bounces, and warming up new IPs gradually.
Can I Use Gmail for SMTP Relay?
Yes, but there are limits. Free Gmail accounts max out at 500 emails per day. Google Workspace bumps it to 2,000. For more, use a real SMTP provider.
Is SMTP Relay Free?
Somehow. Some providers give free plans, but limits are tight. SendGrid, for example, offers 100 emails per day, Mailgun gives 5,000 per month (for three months), and Gmail caps at 500 daily. If you send a lot of emails, you’ll need a paid plan.
How Can I Secure My SMTP Relay?
Security isn’t optional—it’s a must. Always protect it with strong authentication like passwords, API keys, or OAuth. Also, encrypt emails with TLS to prevent interception. Restrict access with IP whitelisting. And don't forget to set up SPF, DKIM, and DMARC to block spoofers.
