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Before you can receive emails with Lettr, you need to create an inbound domain with a forwarding URL and set up MX records to route incoming mail to Lettr’s servers. A domain receives mail only when both are in place: the MX records bring mail to Lettr, and the forwarding URL is where Lettr delivers it.

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Prerequisites

Before setting up inbound email, ensure you have:
  • A verified Lettr account
  • Access to your domain’s DNS settings
  • An HTTP endpoint that accepts POST requests and returns a 2xx status. This is your forwarding URL, and it is required to create the domain.

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Step-by-Step Setup

1

Create an Inbound Domain

Navigate to Domains → Inbound in your dashboard and click Create Inbound Domain. Enter the domain or subdomain you want to use for receiving emails, and the Forwarding URL where Lettr should deliver the mail it receives.
2

Configure MX Records

Add the provided MX records to your domain’s DNS settings. These records tell email servers to route mail to Lettr.
3

Verify MX Records

Click Verify MX Records on the domain’s page to confirm your MX records are properly configured. Once they verify, the domain shows This domain is receiving mail.

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Adding an Inbound Domain

  1. Go to Domains → Inbound
  2. Click Create Inbound Domain (New Inbound Domain once you already have one)
  3. Enter your domain in Domain Name (e.g., mail.example.com)
  4. Enter your endpoint in Forwarding URL (e.g., https://yourapp.com/webhooks/inbound). It must be a full http:// or https:// address.
  5. Optionally enter an Auth token. Lettr sends it to your endpoint in the X-MessageSystems-Webhook-Token header so you can verify each request.
  6. Click Create Inbound Domain
Before the domain is created, your forwarding URL is tested with a POST request (sent by SparkPost, the mail infrastructure behind Lettr’s inbound servers). The endpoint must answer with a 2xx status. If it doesn’t, the dashboard shows the status your endpoint returned, for example “Your forwarding URL answered SparkPost with HTTP 403”, and the domain is not created, so you can fix the endpoint and try again. Once the domain is created, the dashboard displays the MX records you need to add.
Endpoints behind Cloudflare or another firewall/WAF often answer the test request with a challenge page (for example HTTP 403). Allow server-to-server POST requests on your forwarding URL’s path. SparkPost publishes no fixed source IP addresses, so use the auth token to verify requests instead of an IP allowlist. See Security.
Inbound domain management is available through the Lettr dashboard. There is no public API endpoint for creating or managing inbound domains.

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DNS Configuration

Add these MX records to your domain’s DNS settings:
If you’re using a subdomain like mail.example.com, set the Host/Name field to mail instead of @.

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Understanding MX Priority

All three MX records use the same priority (10), which provides load balancing: With equal priority, sending servers distribute email across all three servers. If one is unavailable, the others handle delivery automatically.

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Verifying Your Domain

After adding MX records, verify they’re properly configured:
  1. Go to Domains → Inbound and open your domain
  2. Click Verify MX Records
  3. Wait for the verification to complete
If the MX check fails because the domain has no MX records, the dashboard tells you where mail sent to the domain is actually going. A domain without MX records falls back to its A record, so mail lands on that host (usually a web server) instead of reaching Lettr.

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Domain Status

Each inbound domain shows one status that tells you whether it is receiving mail, or what is missing. In the Inbound Domains list: A domain can show both MX not verified and No forwarding URL. The domain’s page shows the same verdict as This domain is receiving mail or This domain is not receiving mail yet, with each missing piece listed.

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Mail Forwarding

The domain’s page has a Mail Forwarding card where you can add or change the forwarding URL and auth token, then click Save forwarding URL. The new URL is tested the same way as when the domain is created. Leave the Auth token field blank to keep the token that is already set.
Inbound domains created before forwarding URLs were required have no forwarding URL, so mail sent to them is accepted and then discarded. Their Mail Forwarding card shows Not configured. Add a URL there to start receiving mail.

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Using Subdomains vs Root Domain

We recommend using a subdomain for inbound email:
Using subdomains lets you keep your root domain’s MX records pointing to your regular email provider (Gmail, Microsoft 365, etc.) while routing specific addresses through Lettr.

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DNS Propagation

DNS changes can take time to propagate:
If verification fails immediately after adding records, wait 15-30 minutes and try again. Use a tool like MXToolbox to check your MX records externally.

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Next Steps

Email Parsing

Understand the payload posted to your forwarding URL

Routing

Route emails to different handlers

Attachments

Handle incoming attachments

Inbound Domains

Learn more about domain configuration