What Is an SMTP Relay?

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.

Most applications should not deliver email directly. Direct delivery requires managing IP reputation, handling bounces, implementing retry logic, and maintaining TLS certificates. An SMTP relay handles all of this.

How an SMTP Relay Works

1. Your application connects to the relay server via SMTP (port 587 or 465)
2. The relay authenticates your connection using username and password
3. Your application submits the email message
4. The relay queues the message and signs it with DKIM
5. The relay delivers the message to the recipient’s mail server
6. If delivery fails temporarily, the relay retries with exponential backoff

When to Use an SMTP Relay

• Your application framework has built-in SMTP support (Rails, Django, Laravel, WordPress)
• You want to switch email providers without changing application code
• You need to integrate legacy systems that only support SMTP
• You want managed deliverability without building your own mail infrastructure

SMTP Relay vs Direct Delivery

Aspect	SMTP Relay	Direct Delivery
IP reputation	Managed by the relay provider	You manage your own IPs
Bounce handling	Automatic retry and suppression	You build retry logic
DKIM signing	Handled by the relay	You manage keys and signing
Deliverability	Optimized by the provider	Depends on your infrastructure
Setup	Change SMTP config	Build and maintain a mail server

RelayPost SMTP Relay

RelayPost provides SMTP relay on port 587 (STARTTLS) and 465 (TLS). Create SMTP credentials from the dashboard and update your application’s SMTP settings. See the Send via SMTP guide for configuration examples in Nodemailer, Rails, Django, and Laravel.

In short

An SMTP relay forwards your email through managed infrastructure with built-in authentication, retry logic, and deliverability optimization. Use it when your app already speaks SMTP.

Frequently Asked Questions

What is the difference between an SMTP relay and an SMTP server?

An SMTP server is any server that handles email via the SMTP protocol. An SMTP relay is specifically an intermediary server that forwards email from your application to the recipient’s mail server. The relay handles authentication, queuing, and retry logic on your behalf.

Do I need an SMTP relay to send email?

You do not strictly need one, but it is strongly recommended. Without a relay, your application must handle IP reputation, bounce processing, retry logic, and TLS certificates directly. An SMTP relay manages all of this, significantly improving deliverability.

Is SMTP relay the same as an email API?

No. An SMTP relay uses the SMTP protocol (port 587/465) and works with any application that has SMTP configuration. An email API uses HTTPS REST requests with JSON payloads. Both deliver through the same infrastructure — the choice depends on your integration preference.