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Prerequisites

To get the most out of this guide, you’ll need to: Prefer watching a video? Check out our video walkthrough below.

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SMTP Credentials

When configuring your SMTP integration, you’ll need to use the following credentials:
  • Host: smtp.resend.com
  • Port: 25, 465, 587, 2465, or 2587
  • Username: resend
  • Password: YOUR_API_KEY
Ports help to instruct the type of security you want to use in your SMTP connection.
TypePortSecurity
SMTPS465, 2465Implicit SSL/TLS (Immediately connects via SSL/TLS)
STARTTLS25, 587, 2587Explicit SSL/TLS (First connects via plaintext, then upgrades to SSL/TLS)

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Idempotency Key

Idempotency keys are used to prevent duplicate emails. You can add the Resend-Idempotency-Key header to your emails sent with SMTP to prevent duplicate emails.
SMTP
From: Acme <onboarding@resend.dev>
To: delivered@resend.dev
Subject: hello world
Resend-Idempotency-Key: welcome-user/123456789

<p>it works!</p>
Learn more about idempotency keys.

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Custom Headers

If your SMTP client supports it, you can add custom headers to your emails. Here are some common use cases for custom headers:
  • Prevent threading on Gmail with the X-Entity-Ref-ID header
  • Include a shortcut for users to unsubscribe with the List-Unsubscribe header

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FAQ

Once configured, you should be able to start sending emails via SMTP. Below are some frequently asked questions:
We currently don’t provide SMTP server logs for debugging. If you run into issues, please reach out to support.
Emails sent with SMTP will show in your emails table.
Yes, the rate limit is the same as the API.
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