Lettr

Send through Lettr's SaaS email API with CC, BCC, reply-to, headers, metadata, a single tag, attachments, and scheduled delivery.

Capabilities

Repeated headers	Idempotency	Scheduling	Personalized
No	`none`	Yes	`expanded`

These values come from the adapter's exported capabilities declaration. The field support matrix covers normalized message fields.

The Lettr adapter calls Lettr's POST /emails endpoint with plain fetch. Scheduled messages use POST /emails/scheduled instead. Lettr is a European email platform for SaaS products. It authenticates with a Bearer API key.

Lettr@opencoredev/email-sdk/lettr

Provider docs

Configure

Create a Lettr API key (prefixed lttr_ or lttr_sandbox_) and verify the sending domain for live keys. Sandbox keys rewrite the sender domain to dev.uselettr.com and deliver to the key owner's inbox.

lib/email.ts

import { createEmailClient } from "@opencoredev/email-sdk";
import { lettr } from "@opencoredev/email-sdk/lettr";

export const email = createEmailClient({
  adapters: [lettr({ apiKey: process.env.LETTR_API_KEY! })],
});

Prop

Type

Send

Lettr accepts multiple recipients, CC, BCC, one reply-to, custom headers, metadata, one tag, and base64 attachments. Display names on from and replyTo map to from_name and reply_to_name. Recipient addresses are sent as bare emails.

const result = await email.send({
  from: "Acme <hello@acme.com>",
  to: "user@example.com",
  subject: "Welcome to Acme",
  html: "<p>Thanks for joining Acme.</p>",
  text: "Thanks for joining Acme.",
});

console.log(result.id); // Lettr request_id

One tag, one reply-to, no inline images

Lettr stores a single tag string per email. The adapter sends the first tag's value and throws if a message has more than one tag, more than one reply-to, more than 10 custom headers, an inline attachment, or a recipient display name. Combined to, cc, and bcc cannot exceed 50 addresses.

From a verified sending domain

Live keys require a verified sender domain. Sandbox keys skip that check and rewrite the domain to dev.uselettr.com.

Schedule a send

Pass sendAt to use Lettr's scheduled endpoint. Lettr requires the time to be at least five minutes in the future and within three days.

await email.send({
  from: "Acme <hello@acme.com>",
  to: "user@example.com",
  subject: "Reminder",
  text: "Don't forget.",
  sendAt: new Date("2026-09-01T09:00:00.000Z"),
});

Verify from the CLI

LETTR_API_KEY="lttr_..." npx email-sdk doctor --adapter lettr

LETTR_API_KEY="lttr_..." npx email-sdk send \
  --adapter lettr \
  --from "Acme <hello@acme.com>" \
  --to user@example.com \
  --subject "Lettr smoke test" \
  --text "It works" \
  --dry-run

Drop --dry-run to send for real. Use a sandbox key when you want the message delivered to your own inbox.

Capabilities

Configure

Send

Schedule a send

Verify from the CLI

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