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Configure Supabase Auth to send transactional emails (confirmations, password resets, magic links, etc.) through Lettr using the built-in SMTP integration. No code changes required — just add your SMTP credentials in the Supabase dashboard.

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Prerequisites

Before you begin, make sure you have:

API Key

Create an API key in the Lettr dashboard

Verified Domain

Add and verify your sending domain
You’ll also need:
  • A Supabase project
  • A sender address on your verified domain (e.g. noreply@yourdomain.com)

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Get Your SMTP Credentials

You’ll use the following Lettr SMTP settings when configuring Supabase:
SettingValue
Hostsmtp.lettr.com
Port465
Usernamelettr
PasswordYour API key (starts with lttr_)

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Configure Supabase Auth

1

Open SMTP settings in Supabase

  1. Go to your Supabase project dashboard
  2. Click Authentication in the left sidebar
  3. Click SMTP Settings under the Email provider section
  4. Toggle Enable Custom SMTP on
2

Set sender details

Fill in your sender information:
  • Sender email: Your verified sending address (e.g. noreply@yourdomain.com)
  • Sender name: Your application name (e.g. My App)
The sender email must use a domain you’ve verified in the Lettr dashboard. Emails from unverified domains will be rejected.
3

Enter SMTP credentials

Enter the Lettr SMTP credentials:
  • Host: smtp.lettr.com
  • Port number: 465
  • Username: lettr
  • Password: Your Lettr API key (starts with lttr_)
4

Save

Click Save to apply the configuration. All Supabase Auth emails — confirmations, password resets, magic links, and invites — will now be sent through Lettr.

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Verify It Works

After saving, trigger a test email to confirm everything is connected:
  1. Go to Authentication → Users in your Supabase dashboard
  2. Click Add user → Send invitation
  3. Enter your own email address and send the invite
  4. Check your inbox for the invitation email
You can also verify delivery in the Lettr logs, where the email should appear within seconds.
If you don’t see the email, check the Troubleshooting section below or review your Lettr logs for error details.

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Custom Email Templates

Supabase lets you customize the HTML templates for each auth email type (confirmation, invite, magic link, password reset). These templates are sent through Lettr as-is. To edit templates:
  1. Go to Authentication in your Supabase dashboard
  2. Click Email Templates
  3. Select the template type you want to customize
  4. Edit the HTML and save
Supabase email templates use Go template syntax with variables like {{ .ConfirmationURL }}. These are processed by Supabase before the email is handed off to Lettr for delivery.

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Troubleshooting

  • Verify the sender email uses a domain verified in Lettr
  • Double-check that the host is smtp.lettr.com and the port is 465
  • Ensure your API key is correct and starts with lttr_
  • Confirm the username is exactly lettr (lowercase)
Supabase applies its own rate limits on auth emails (e.g. one confirmation per 60 seconds per address). These are separate from Lettr’s rate limits. Check your Supabase Auth settings under Rate Limits.
Make sure your sending domain has proper DNS records configured:
  • SPF — Authorizes Lettr to send on your behalf
  • DKIM — Cryptographically signs your emails
  • DMARC — Tells receivers how to handle unauthenticated mail
See Deliverability Best Practices for more tips.
If port 465 doesn’t work in your environment, you can use port 587 with STARTTLS instead. Both are secure — port 465 establishes an encrypted connection immediately, while 587 upgrades via STARTTLS.

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What’s Next

SMTP Introduction

Learn more about Lettr’s SMTP service

Sending Domains

Verify your sending domain

Deliverability

Improve email deliverability

Troubleshooting

Fix common delivery issues

Supabase Integration

Full integration guide with Edge Functions