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Send transactional emails through Lettr’s SMTP relay server. If your application already uses SMTP or you’re migrating from another email provider, you can switch to Lettr without changing your code — just update your SMTP credentials. Using Cursor? Jump straight in using this prompt

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Why Use SMTP?

SMTP is a good choice when:
  • Legacy applications — Your application already uses SMTP and changing to an HTTP API would require significant refactoring
  • Framework built-ins — Your framework has built-in mail functions that expect SMTP credentials (WordPress, Django, Rails, etc.)
  • Third-party integrations — You’re connecting a CMS, CRM, or other tool that only supports SMTP configuration
  • Gradual migration — You’re migrating from another email provider and want to minimize code changes
If you’re building a new application or have flexibility in your stack, consider using the HTTP API or one of our SDKs instead. The API offers better error handling, request tracking, and doesn’t require maintaining a persistent connection.

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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

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SMTP Credentials

Use the following settings to connect to Lettr’s SMTP server:
Your SMTP password is your Lettr API key. Never commit this to version control or share it publicly. Store it in environment variables or a secrets manager.

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Ports & Security

The port you choose determines the security protocol for your SMTP connection: We recommend using port 465 with implicit TLS for the best security and compatibility. Port 587 with STARTTLS is also widely supported.
If you’re behind a restrictive firewall, port 2465 and 2587 are alternate ports that often work when standard ports are blocked.

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Quick Test with swaks

Verify your SMTP connection using swaks (Swiss Army Knife for SMTP):
If the connection succeeds, you’ll see output showing the SMTP conversation and a confirmation that the message was accepted.
swaks is available via package managers: brew install swaks (macOS), apt-get install swaks (Ubuntu/Debian), or download from jetmore.org/john/code/swaks.

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Framework Integration Guides

Choose your framework or library for specific setup instructions:

Laravel

Configure Laravel’s mail system with SMTP

PHPMailer

Send emails with PHPMailer

Supabase

Send Supabase Auth emails through Lettr

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Troubleshooting

If you see “Authentication failed” or “535 Authentication credentials invalid”:
  • Verify your API key is correct and starts with lttr_
  • Check that the username is exactly lettr (lowercase)
  • Ensure your API key hasn’t been revoked in the dashboard
If the connection times out:
  • Check your firewall isn’t blocking outbound connections on the SMTP port
  • Try alternate ports (2465 or 2587)
  • Verify smtp.lettr.com resolves correctly: nslookup smtp.lettr.com
If you see TLS or SSL handshake errors:
  • Ensure you’re using the correct encryption setting for your port (implicit TLS for 465, STARTTLS for 587)
  • Update your TLS/SSL libraries if you’re using older versions
  • Some older SMTP clients may not support modern TLS versions — consider upgrading or using the HTTP API instead

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SMTP vs API

Here’s a quick comparison to help you decide:
Both SMTP and the HTTP API deliver emails with the same reliability and deliverability. The choice depends on your application’s needs and existing infrastructure.

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What’s Next

Laravel SMTP Setup

Configure Laravel to send via SMTP

PHPMailer Setup

Use PHPMailer with Lettr SMTP

Supabase Setup

Send Supabase Auth emails through Lettr

API Reference

Explore the HTTP API alternative

WordPress

Send WordPress emails through Lettr

Integrations

Stripe, Supabase, and more