How to Switch from Resend to SMTPfast in 5 Minutes

Why Switch?

SMTPfast offers a Resend-compatible API, so you can migrate without rewriting your email integration. You get the same developer experience with competitive pricing and full delivery tracking.

2 lines

The only code changes you need to make

Before and After

Before (Resend with fetch)

const res = await fetch("https://api.resend.com/emails", {
  method: "POST",
  headers: {
    Authorization: "Bearer re_your_resend_key",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    from: "[email protected]",
    to: "[email protected]",
    subject: "Welcome!",
    html: "<h1>Hello!</h1>",
  }),
});

const { id } = await res.json();

After (SMTPfast)

const res = await fetch("https://smtpfa.st/api/v1/emails", {
  method: "POST",
  headers: {
    Authorization: "Bearer sf_your_smtpfast_key",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    from: "[email protected]",
    to: "[email protected]",
    subject: "Welcome!",
    html: "<h1>Hello!</h1>",
  }),
});

const { id } = await res.json();

That is it. Two values changed: the base URL and the API key. Everything else stays the same.

Using the Resend Node SDK?

If you are using the Resend SDK, you can point it at SMTPfast by overriding the base URL:

import { Resend } from "resend";

const resend = new Resend("sf_your_smtpfast_key");

// Override the base URL to point to SMTPfast
resend.baseUrl = "https://smtpfa.st/api/v1";

await resend.emails.send({
  from: "[email protected]",
  to: "[email protected]",
  subject: "Welcome!",
  html: "<h1>Hello!</h1>",
});

Or use a simple fetch wrapper if you prefer not to depend on the Resend SDK at all.

Compatible Fields

SMTPfast accepts all the common Resend email fields:

Field	Type	Notes
`from`	`string`	Required. Sender address.
`to`	`string \| string[]`	Required. One or more recipients.
`cc`	`string \| string[]`	Optional. Carbon copy recipients.
`bcc`	`string \| string[]`	Optional. Blind carbon copy recipients.
`reply_to`	`string \| string[]`	Optional. Reply-to address(es).
`subject`	`string`	Required. Email subject line.
`html`	`string`	Email body as HTML.
`text`	`string`	Email body as plain text.
`tags`	`Array<{ name, value }>`	Optional. Metadata tags.
`headers`	`Record<string, string>`	Optional. Custom email headers.
`scheduled_at`	`string`	Optional. ISO 8601 date, up to 30 days ahead.
`template`	`{ id, variables }`	Optional. Send a published template by id or alias instead of `html`/`text`.

Response Format

SMTPfast returns a Resend-compatible response:

{ "id": "clxyz123abc" }

You can retrieve full delivery details with GET /emails/:id, which returns Resend-compatible fields like last_event, from, to, subject, html, text, and created_at.

Retrieving Email Status

const res = await fetch("https://smtpfa.st/api/v1/emails/clxyz123abc", {
  headers: { Authorization: "Bearer sf_your_smtpfast_key" },
});

const email = await res.json();
// email.last_event -> "delivered"
// email.created_at -> "2026-04-07T12:00:00.000Z"

Migration Checklist

Create an SMTPfast account at smtpfa.st
Add and verify your sending domain in the SMTPfast dashboard
Generate an API key with email:send scope
Replace your base URL from https://api.resend.com to https://smtpfa.st/api/v1
Replace your API key from re_... to sf_...
Test by sending a single email and confirming delivery

The whole process takes about 5 minutes. Your existing code, payload structure, and response handling all stay the same.

Beyond Sending

More of Resend's API works against SMTPfast with the same field names:

Templates: create, publish and send templates by id or alias. See the Templates docs.
Inboxes: threads, labels, drafts, reply and forward on your own domain. See the Inboxes docs.
Contacts and segments, suppressions in bulk, broadcast recipients and email metrics.
Webhook events: list deliveries, see each attempt, replay an event and rotate the signing secret.

What About Webhooks?

SMTPfast supports delivery webhooks for events like delivered, bounced, opened, and clicked. The webhook payload format may differ slightly from Resend. Check the API docs for details on configuring webhooks.