Smtpfast

SMTPfast API

Send email and manage sending from SMTPfast, a transactional email API. This skill gives you the auth model, the endpoints, and copy-ready request examples so an agent can integrate SMTPfast correctly on the first try.

When to use this

Trigger on requests like:

• "Send an email with SMTPfast" / "use the smtpfa.st API"
• "Add transactional email to this app" (when SMTPfast is the provider)
• "Check whether that email was delivered"
• "Create a sending domain / contact / broadcast on SMTPfast"
• "Wire up an SMTPfast webhook"

The essentials

• Base URL: https://smtpfa.st/api
• API version: all endpoints are under /v1, so a full URL looks like https://smtpfa.st/api/v1/emails.
• Auth: a Bearer token on every request. Create an API key in the SMTPfast dashboard and send it as Authorization: Bearer sf_live_.... Never hardcode it; read it from an environment variable (SMTPFAST_API_KEY).
• Content type: application/json.
• Sending domain: the from address must belong to a domain you have verified in SMTPfast. If a send fails with a domain error, verify the domain first (see the Domains endpoints).

Send an email

The one call you need most. Required fields: from, to (an array), subject. Common optional fields: html, text, cc, bcc, reply_to, headers, tags.

curl -X POST https://smtpfa.st/api/v1/emails \
  -H "Authorization: Bearer $SMTPFAST_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "[email protected]",
    "to": ["[email protected]"],
    "subject": "Welcome!",
    "html": "<h1>Hello!</h1><p>Thanks for signing up.</p>"
  }'

A successful call returns an Email object with an id (e.g. email_abc123) and a status. The email is queued and sent asynchronously, so status starts as queued; poll the email by id to see it progress to delivered.

// Node 18+ (built-in fetch)
const res = await fetch("https://smtpfa.st/api/v1/emails", {
  method: "POST",
  headers: {
    Authorization: `Bearer ${process.env.SMTPFAST_API_KEY}`,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    from: "[email protected]",
    to: ["[email protected]"],
    subject: "Welcome!",
    html: "<h1>Hello!</h1>",
  }),
});
if (!res.ok) throw new Error(`SMTPfast ${res.status}: ${await res.text()}`);
const email = await res.json(); // { id: "email_...", status: "queued", ... }

More languages (Python, PHP) are in examples/send-email.md.

Unsubscribe links (marketing and bulk mail)

For any non-essential mail, include the placeholder {{unsubscribe_url}} anywhere in your html (or text). SMTPfast substitutes the per-recipient unsubscribe link at send time and automatically sets the RFC 8058 List-Unsubscribe header. This keeps you compliant and helps deliverability.

{ "html": "<p>...</p><p><a href=\"{{unsubscribe_url}}\">Unsubscribe</a></p>" }

Send many at once

To send up to 100 emails in one request, POST a JSON array of the same email objects to /v1/emails/batch. Validation runs on every item first: one malformed item rejects the whole batch with 400. Suppressed recipients are dropped per row.

Check delivery status

curl https://smtpfa.st/api/v1/emails/email_abc123 \
  -H "Authorization: Bearer $SMTPFAST_API_KEY"

Returns the email's current status and its delivery events (queued, sent, delivered, bounced, complained, opened, clicked). Prefer webhooks over polling for anything real-time (see below).

The rest of the API

Full endpoint list with methods and parameters is in references/api-reference.md. Load it when the task goes beyond sending. In brief:

• Contacts (/v1/contacts): create/list/update contacts, export, and organize them.
• Segments (/v1/segments): group contacts for targeting.
• Suppressions (/v1/suppressions): manage the do-not-send list (unsubscribes, bounces, complaints).
• Broadcasts (/v1/broadcasts): create a campaign, send a test, then send or cancel it.
• Domains (/v1/domains): add a sending domain and trigger DNS verification.
• Webhooks (/v1/webhooks): subscribe to delivery events; the preferred way to track status.
• API keys (/v1/api-keys) and Analytics (/v1/analytics).

Handling responses and errors

• 2xx: success. Sends return 200 with the queued Email (or batch result).
• 400: bad request (missing from/to/subject, malformed body, unverified domain). Read the error message; do not retry blindly.
• 401 / 403: bad or missing API key, or the key lacks permission. Fix the credential.
• 429: rate limited. Back off and retry with exponential delay.
• 5xx: transient server error. Retry a few times with backoff.

Always check res.ok (or the status code) and surface the response body on failure; the API returns a descriptive error message.

Good habits

• Keep the API key in an env var, never in code or logs.
• Send a text fallback alongside html for deliverability.
• Use webhooks, not polling loops, to react to delivery events.
• Include {{unsubscribe_url}} on anything that is not strictly transactional.
• On 429/5xx, retry with exponential backoff; on 400/401, fix the request rather than retrying.

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By SMTPfast. See the full interactive API reference and dashboard at smtpfa.st. Built with help from DevOps Daily.