Custom headers

headers is an object of string keys to string values, written into the message as-is.

Node.js
cURL

import { SpacePost } from "@spacepost/node";

const spacepost = new SpacePost(process.env.SPACEPOST_API_KEY);

await spacepost.emails.send({
  from: "Acme <hello@acme.com>",
  to: "user@example.com",
  subject: "Weekly digest",
  html: "<p>...</p>",
  headers: {
    "List-Unsubscribe": "<https://acme.com/unsubscribe?u=1042>, <mailto:unsubscribe@acme.com>",
    "List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
    "X-Entity-Ref-ID": "digest-2026-w43-1042",
  },
});

curl -X POST https://api.spacepost.dev/emails \
  -H "Authorization: Bearer $SPACEPOST_API_KEY" -H "Content-Type: application/json" \
  -d '{
    "from": "Acme <hello@acme.com>",
    "to": ["user@example.com"],
    "subject": "Weekly digest",
    "html": "<p>...</p>",
    "headers": {
      "List-Unsubscribe": "<https://acme.com/unsubscribe?u=1042>",
      "List-Unsubscribe-Post": "List-Unsubscribe=One-Click"
    }
  }'

Common uses

Header	Purpose
`List-Unsubscribe` and `List-Unsubscribe-Post`	One-click unsubscribe (RFC 8058). Required by Gmail and Yahoo for bulk senders. Broadcasts add it automatically.
`X-Entity-Ref-ID`	A unique value per email. Stops Gmail from collapsing similar messages into one thread or hiding the footer.
`In-Reply-To`, `References`	Thread a message under an existing conversation.
`X-*`	Any application-specific metadata. Over SMTP, `X-` headers are forwarded, except SpacePost’s own `X-SpacePost-` control headers.