Rather than inlining HTML on every send, save a template once and reference it by ID. Pass a data object and any {{variable}} in the template is substituted at send time.

curl https://www.unitpost.com/api/v1/emails \
  -H "Authorization: Bearer $API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "you@yourdomain.com",
    "to": "customer@acme.com",
    "template": { "id": "tmpl_123" },
    "data": { "first_name": "Sam", "plan": "Pro" }
  }'

When you send to a known contact, that contact's custom fields merge into the variables automatically — so a template can reference {{first_name}} without you re-sending it on every request.

Templates are authored in the visual editor or in code mode with the @unitpost/email component library — manage them under Templates. The library — every block, its props, and copy-paste snippets — is documented under the Email section in this sidebar.