What is an SMTP relay?
An SMTP relay is a mail server that forwards email messages from a sender's application to the recipient's mail server. Instead of delivering email directly, the relay acts as an intermediary that handles authentication, queue management, and retry logic, improving deliverability for applications that send high volumes of transactional or marketing email.
RelayPost's SMTP relay accepts email from your application, signs it with DKIM, manages delivery queues, handles retries for temporary failures, and tracks every event from send to inbox.
Quick setup
Point your application's SMTP settings at RelayPost. No SDK installation or code changes required.
SMTP Host: smtp.relaypost.dev
SMTP Port: 587 (STARTTLS) or 465 (TLS)
Username: your-smtp-username
Password: your-smtp-password
Encryption: STARTTLS (port 587) or TLS (port 465)Generate SMTP credentials from your dashboard settings after signing up.
SMTP vs API: which to use
| Feature | SMTP Relay | REST API |
|---|---|---|
| Protocol | SMTP (port 587 or 465) | HTTPS (REST with JSON) |
| Authentication | Username and password | API key in Authorization header |
| Integration effort | Change SMTP config — no code | Add HTTP calls to your app |
| Speed | Connection overhead per session | Single HTTP request per email |
| Features | Basic send with attachments | Templates, scheduling, priority |
| Best for | WordPress, Rails, Django, legacy apps | Modern apps, serverless, high volume |
| Deliverability | Same infrastructure | Same infrastructure |
Both methods use the same delivery infrastructure. Choose based on your application's needs. You can use both simultaneously.
Works with everything
WordPress
Use WP Mail SMTP or Post SMTP plugin. Enter RelayPost credentials and all WordPress emails route through our relay.
Ruby on Rails
Update config/environments/production.rb with RelayPost SMTP settings. Works with Action Mailer out of the box.
Django
Set EMAIL_HOST, EMAIL_PORT, and credentials in settings.py. Django's email backend handles the rest.
Node.js (Nodemailer)
Pass RelayPost SMTP config to Nodemailer's createTransport(). Compatible with any Nodemailer-based setup.
PHP
Works with PHPMailer, SwiftMailer, and Laravel Mail. Update your SMTP configuration and you're set.
IoT & embedded
Any device or system that supports SMTP can send through RelayPost. Monitoring alerts, sensor notifications, system reports.
What you get
Automatic DKIM signing
Every email sent through the relay is automatically signed with DKIM. No manual key management.
Bounce handling
Hard bounces are automatically added to your suppression list. No more sending to dead addresses.
Delivery tracking
Track every email from send to inbox. Delivery, bounce, open, and complaint events available via webhooks and dashboard.
TLS encryption
All SMTP connections are encrypted with STARTTLS (port 587) or implicit TLS (port 465). Email content is never transmitted in plaintext.
SMTP ports explained
| Port | Encryption | Status | Use case |
|---|---|---|---|
| 587 | STARTTLS | Recommended | Standard submission port for authenticated email |
| 465 | Implicit TLS | Supported | Legacy TLS port, re-standardized in RFC 8314 |
| 25 | None/STARTTLS | Not supported | Server-to-server relay only, blocked by most cloud providers |
| 2525 | STARTTLS | Not supported | Unofficial alternative when 587 is blocked |
Use port 587 with STARTTLS for the best compatibility. See our SMTP ports guide for details.
Frequently asked questions
What is an SMTP relay?
An SMTP relay is a mail server that forwards email messages from a sender's application to the recipient's mail server. Instead of delivering email directly, the relay acts as an intermediary that handles authentication, queue management, and retry logic, improving deliverability for applications that send high volumes of transactional or marketing email.
Why use an SMTP relay instead of sending directly?
Sending email directly from your application server means managing IP reputation, handling bounces, retrying failed deliveries, and configuring authentication. An SMTP relay handles all of this for you, plus provides delivery tracking, suppression lists, and analytics.
Which SMTP port should I use?
Use port 587 with STARTTLS (recommended) or port 465 with implicit TLS. Port 25 is blocked by most cloud providers and ISPs. Both 587 and 465 require authentication with your SMTP credentials.
Can I use SMTP relay with WordPress?
Yes. Install a WordPress SMTP plugin (like WP Mail SMTP or Post SMTP), enter your RelayPost SMTP credentials, and all WordPress emails will route through RelayPost. No code changes needed.
What is the difference between SMTP relay and email API?
SMTP relay uses the SMTP protocol (port 587/465) and works with any application that can send email. An email API uses HTTP/REST and requires code changes. SMTP is easier for legacy apps; API offers more features like templates and scheduling.
Start using SMTP relay
Free tier includes 1,000 emails per month. Set up in under 5 minutes.
Create free account