SMTP relays make bulk delivery much more reliable, but configuration and complexity can make setup difficult. Below, we’ll explain what an SMTP relay is, how it works, and how you can quickly set one up for yourself.

An SMTP relay is a service that helps send emails from one server to another. It acts as a middleman, passing your email through the internet to the recipient’s server. This is especially useful for sending bulk emails or messages from custom domains. Businesses use SMTP relays to ensure their emails are delivered reliably without getting flagged as spam. It’s a critical component for anyone managing high-volume email communication.

How an SMTP Relay Works

1. Submission of the Email The SMTP relay process begins when a sender’s email client or application submits an email to the SMTP relay server. The email contains essential information, including the sender’s email address, the recipient’s email address, the subject line, and the message body. The client communicates with the relay server through a predefined SMTP port, typically 587 (for submissions with authentication) or 465 (for encrypted connections using SSL/TLS). Authentication credentials are required to validate the sender and prevent unauthorized access. Once validated, the relay server accepts the email for further processing.
2. Domain Name Resolution After accepting the email, the SMTP relay server identifies the recipient’s email domain (e.g., example.com) by parsing the recipient’s email address. To route the email correctly, the server performs a DNS query to retrieve the Mail Exchange (MX) records associated with the domain. These MX records specify the servers responsible for handling email for the recipient’s domain and their priority levels. The relay server prioritizes contacting the server with the highest priority (lowest numerical value in the MX record).
3. Establishing a Connection to the Recipient’s Server The SMTP relay server initiates a connection to the recipient’s mail server using the MX records obtained during the DNS query. This connection is established over port 25 or another specified SMTP port. During this phase, the relay, and recipient’s servers perform a handshake using SMTP commands like HELO/EHLO (introducing the relay server), MAIL FROM (identifying the sender), and RCPT TO (verifying the recipient’s address). The recipient’s server may also perform security checks, including SPF, DKIM, or DMARC validation, to ensure the legitimacy of the sender.
4. Transmitting the Email Data Once the recipient’s server accepts the RCPT TO command, the SMTP relay sends the email data using the DATA command. This includes the headers (such as “From,” “To,” “Date,” and “Subject”) and the body of the email. The server transmits the data in chunks, adhering to the SMTP protocol’s formatting rules, such as using a single dot (.) on a line to signal the end of the message. The recipient’s server processes the email data and either accepts it for delivery to the recipient’s mailbox or performs additional checks for spam or malware.
5. Handling Delivery and Failures If the recipient’s server accepts the email, it sends a 250 OK response, confirming successful delivery. However, if delivery fails due to an invalid recipient address, server unavailability, or other issues, the SMTP relay generates a Non-Delivery Report (NDR), often referred to as a bounce message. This message is sent back to the sender and includes details about the failure, such as the error code and description. For temporary failures, the relay server may attempt to resend the email at intervals defined by its retry policy before marking it as undeliverable.

How To Configure Your Own SMTP Relay

Step 1: Choose and Install an SMTP Server

Select an SMTP server software like Postfix, Microsoft Exchange, or Exim based on your operating system and requirements. Install the software on a dedicated server or virtual machine to ensure reliable performance. For example, on a Linux-based system, you can install Postfix using the package manager (sudo apt install postfix for Ubuntu). Ensure the server is up-to-date with the latest patches.

Step 2: Configure Basic Settings

Edit the main configuration file for your SMTP server. Define the hostname, relay restrictions, and the IP address or domain the server should listen on. In Postfix, you can configure these in the main.cf file, specifying myhostname (server’s hostname), inet_interfaces (to bind the server), and mydestination (domains the server accepts mail for). Restart the SMTP service after making changes.

Step 3: Set Up Authentication

Enable SMTP authentication to ensure only authorized users can send emails. Configure SASL for user authentication by linking your SMTP server to a local or external authentication system. For example, in Postfix, you can configure SASL with smtpd_sasl_auth_enable=yes and provide a database of valid user credentials. Test the setup to confirm that only authenticated users can relay emails.

Step 4: Secure the Relay with TLS

Configure your SMTP server to enforce encryption for email transmissions. Obtain an SSL/TLS certificate from a trusted Certificate Authority (CA) or use a tool like Let’s Encrypt to generate one. Update your configuration to point to the certificate and enable secure connections. For example, in Postfix, add smtpd_tls_cert_file and smtpd_tls_key_file directives in the main.cf file to specify your certificate and key.

Step 5: Configure Domain-Based Authentication

Set up SPF, DKIM, and DMARC to authenticate your domain and protect against spoofing. Generate a DKIM key pair and add the public key to your DNS as a TXT record. Configure SPF to list your server’s IP addresses and domains allowed to send emails. Finally, create a DMARC record with policies for handling unauthorized emails and receiving reports.

Step 6: Test and Monitor Your Relay

Use tools like telnet or third-party email testing services to verify that your SMTP relay is correctly configured. Send test emails to confirm authentication, encryption, and delivery are functioning as expected. Enable logging for all SMTP activity and monitor these logs regularly. Set up alerts for failed authentications or unusual activity to respond quickly to potential issues.

How To Secure Your SMTP Relay

• Require Authentication Configure your SMTP relay to enforce user authentication before sending emails. Use strong password policies and limit access to authenticated users or applications only. Implement multifactor authentication (MFA) where possible to add an extra layer of security. In most SMTP server configurations, such as Postfix or Microsoft Exchange, you can set this by enabling SMTP AUTH with SASL (Simple Authentication and Security Layer).
• Encrypt Email Traffic Ensure all communications with your SMTP relay are encrypted using SSL/TLS. For encryption, configure your server to use ports 465 (for implicit TLS) or 587 (STARTTLS for upgrading plain text to secure). Update your server’s TLS certificate regularly and disable outdated encryption protocols like SSL 3.0 or TLS 1.0. Modern relay servers should only support TLS 1.2 or 1.3 for secure transmission.
• Enable Domain-Based Authentication Protocols Deploy Sender Policy Framework (SPF) to specify which servers are authorized to send emails for your domain. Set up DKIM (DomainKeys Identified Mail) to sign outgoing emails with a cryptographic signature, allowing recipients to verify authenticity. Finally, configure DMARC (Domain-based Message Authentication, Reporting, and Conformance) to define policies for handling spoofed emails and to receive reports on unauthorized usage. These records are configured in your DNS settings.
• Implement Access Control Restrict relay usage to known IP addresses or subnets using IP whitelisting. Deny access to unknown or suspicious IPs to prevent unauthorized use. For additional protection, use firewalls or access control lists (ACLs) to enforce these rules. Set rate limits to restrict the number of emails a user or IP address can send within a specific period to prevent spam or abuse.
• Monitor and Respond to Suspicious Activity Enable detailed logging for all SMTP transactions to track email activity. Regularly analyze logs to detect unauthorized access attempts, unusual email volumes, or other anomalies. Use tools like fail2ban or intrusion detection systems to block IPs showing repeated failed login attempts. Configure alerts for abnormal patterns, such as sudden spikes in outbound email traffic or failed authentications.

SMTP Relay Challenges

• Deliverability Issues SMTP relay servers must be carefully configured to ensure high email deliverability rates. Improperly configured SPF, DKIM, or DMARC records can lead to failures during recipient domain authentication checks. Additionally, if your relay’s IP address is flagged on spam blacklists or the content of your emails triggers spam filters, emails may be rejected or sent to junk folders. Regularly verify DNS records, monitor your IP reputation using tools like MXToolBox, and ensure email content complies with anti-spam standards.
• Abuse and Spam Exploitation SMTP relays are a frequent target for spammers and malicious actors. An improperly secured relay can become an open relay, allowing attackers to send large volumes of spam or phishing emails. This abuse can result in blacklisting, server overload, and legal consequences. Implement strong authentication (e.g., SMTP AUTH), restrict relay access with IP whitelisting, and enforce rate limits to mitigate unauthorized use.
• Security Vulnerabilities SMTP relays face several security risks, including interception of emails, credential theft, and denial-of-service (DoS) attacks. Without TLS encryption, emails can be captured and read during transmission. Outdated software or weak security configurations can leave your relay exposed to known exploits. Use TLS 1.2 or higher for encryption, apply regular updates and patches to the server, and monitor logs for signs of intrusion or abuse attempts.
• Performance and Scaling Constraints Managing high volumes of email traffic presents performance challenges for SMTP relays. Limited hardware resources, such as CPU, memory, or disk I/O, can cause email queues to grow, delaying delivery. Additionally, suboptimal settings like low connection timeouts or insufficient concurrency limits may degrade throughput. To address this, optimize server configurations, implement load balancing, and scale resources to meet demand as email traffic increases.

The Best SMTP Relay Platforms

Using dedicated platforms for your SMTP server can make configuring relays significantly easier and more secure. Below are some of my favorite SMTP platforms that support SMTP relays in various ways.

1. SMTP.com

SMTP.com is a powerful SMTP relay service that ensures high email deliverability with robust tools for monitoring and managing your email campaigns. It offers advanced analytics, API integration, and a reliable infrastructure designed to handle high-volume email traffic. While it doesn’t offer a free plan, its scalability and features make it a top choice for businesses that need a professional-grade email relay service. Its long-standing reputation and easy integration with major platforms further enhance its appeal.

Key Features:

• Reliable Email Delivery: Ensures high deliverability rates with a robust infrastructure.
• Advanced Analytics: Provides detailed insights into email performance metrics such as bounce rates and open rates.
• API Integration: Offers seamless connection to third-party platforms for easier workflow management.
• High Scalability: Supports small businesses to enterprises with the ability to handle high email volumes.
• Reputation Monitoring: Tracks and safeguards your sender reputation for optimal performance.

Why do we recommend it?

SMTP.com is recommended for its reliability, advanced analytics, and seamless API integration. I found that it is an excellent choice for businesses focused on email deliverability. Its scalability is ideal for companies planning to grow their email outreach.

Who is it recommended for?

This service is best suited for businesses and enterprises that prioritize high deliverability rates and require advanced tools to monitor email performance. It’s ideal for those with high email volumes or requiring seamless integration into their existing systems.

2. Mailgun

Mailgun is a feature-rich SMTP relay service tailored for developers and businesses that require precise control over their email delivery. It offers a comprehensive set of APIs, advanced analytics, and exceptional support for transactional and marketing emails. With a limited free plan available, Mailgun provides an accessible entry point for smaller projects while maintaining scalability for growing needs. Its user-friendly interface and strong documentation make it a top choice for those who value customization and performance.

Key Features:

• Powerful Email APIs: Offers highly customizable APIs for full control over email delivery.
• Free Tier Availability: Includes a limited free plan, perfect for testing and small-scale projects.
• Transactional Email Focus: Designed for time-sensitive email deliveries such as receipts or confirmations.
• Detailed Analytics: Provides in-depth metrics on open rates, clicks, and delivery performance.
• Sender Reputation Management: Helps safeguard your IP reputation for consistent deliverability.

Why do we recommend it?

Mailgun stands out for its developer-centric approach, offering powerful APIs and excellent deliverability for transactional and marketing emails. I noted that its free tier and flexible pricing make it a versatile choice for a variety of needs.

Who is it recommended for?

Mailgun is best suited for developers, startups, and businesses requiring detailed control over their email delivery and performance. It’s particularly useful for teams managing transactional emails or those who value scalable and customizable solutions.

3. Elastic Email

Elastic Email is a cost-effective SMTP relay and email marketing service that offers robust tools for managing email campaigns. It provides a wide range of features, including API access, advanced analytics, and built-in tools for creating and sending emails. Elastic Email also offers a free plan for users with low-volume needs, making it an excellent choice for startups or small businesses. Its affordable pricing and versatile features cater to users looking for a comprehensive email solution.

Key Features:

• All-in-One Email Platform: Combines SMTP relay and email marketing tools in one solution.
• Free Plan Available: Offers a limited free tier, ideal for startups and small businesses.
• Detailed Campaign Analytics: Provides insights into opens, clicks, and bounces for optimization.
• High Deliverability Rates: Utilizes robust infrastructure to ensure emails consistently reach recipients.
• Easy Integration Options: Connects seamlessly with popular tools and platforms like WordPress and Zapier.

Why do we recommend it?

Elastic Email is highly recommended for its combination of SMTP relay services and marketing tools, providing a cost-effective and comprehensive email solution. I learned that its robust analytics and high deliverability rates make it ideal for businesses of all sizes.

Who is it recommended for?

Elastic Email is perfect for startups, small businesses, and growing teams looking for an all-in-one email solution that balances affordability with performance. It’s also well-suited for those needing both transactional and marketing email capabilities.