Documentation
Guides for sending with letr.dev, and the API reference.
Guides
- Quickstart
- Send your first email with one request and check what happened to it.
- Sending domains
- Add a domain, publish one DKIM record and let letr.dev verify it.
- Sending email
- Every field of POST /v1/emails, the limits per email, custom headers and safe retries.
- Attachments and inline images
- Attach files as base64 and show images inline with contentId.
- Batch sending
- Queue up to 100 emails in one call, all or nothing.
- SMTP
- Send through smtp.letr.dev from tools that only speak SMTP, with an API key as the password.
- Webhooks
- Event types, payload, signature verification, retries, pausing, test events and transport security.
- API keys
- Permissions, keys limited to domains, rolling a key with a grace period, and why no key manages keys.
- Usage limits and safeguards
- Included recipients, overage and your monthly ceiling, the daily limit, when an account pauses, and the status codes.
- Suppressions
- Which addresses letr.dev stops sending to, how to add or allow one, and how to erase a recipient's address.
- Errors
- Every error code of the public API with its status and meaning.
- MCP server
- Connect Claude, ChatGPT, Cursor or another MCP client to your letr.dev account.
- Billing
- Who you contract with, in which currency, and where invoices are.
- Reliability and support
- What letr.dev commits to, how the status page measures it and where your data lives.
- Questions and answers
- What letr.dev is, what it costs, how the daily limit grows, when an account pauses, where the data is, and how you send.
Migrate
- Migrate from SendGrid
- letr.dev as a SendGrid alternative, with API keys, DNS records, webhook events, SMTP settings and suppressions mapped for the move.
- Migrate from Resend
- letr.dev as a Resend alternative, with API keys, DNS records, webhook events, SMTP settings and suppressions mapped for the move.
- Migrate from Infomaniak, Hostpoint or cyon
- Move an app that sends through a hosted mailbox to smtp.letr.dev, with the hosters' sending limits, the settings to change and the first week.
Compared
- letr.dev as a Resend alternative
- What differs when you pick letr.dev over Resend, what is the same, when Resend fits better, and how to switch.
- letr.dev as a Postmark alternative
- What differs when you pick letr.dev over Postmark, what is the same, when Postmark fits better, and how to switch.
API reference
- API reference
- Every public endpoint with parameters, schemas, responses and code samples.