Resend Compatibility

SMTPfast is API-compatible with Resend. Switch by changing two values.

Overview

The SMTPfast email API accepts the same request format as Resend. If you are already using Resend, you can switch to SMTPfast by updating the base URL and API key. No other code changes are needed.

All Resend-compatible fields (from, to, cc, bcc, reply_to, subject, html, text, tags, headers) work out of the box. Contacts are also available at the Resend-style /contacts endpoints.

How to Switch

Create an SMTPfast account and verify your sending domain

Create an API key in the SMTPfast dashboard

Replace the Resend base URL with https://smtpfa.st/api/v1 and swap your API key

Before and After

Before (Resend)

javascript

import { Resend } from "resend";

const resend = new Resend("re_your_api_key");

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

After (SMTPfast)

javascript

const res = await fetch("https://smtpfa.st/api/v1/emails", {
  method: "POST",
  headers: {
    Authorization: "Bearer sf_live_your_api_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();

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

import requests

res = requests.post(
    "https://smtpfa.st/api/v1/emails",
    headers={"Authorization": "Bearer sf_live_your_api_key"},
    json={
        "from": "[email protected]",
        "to": "[email protected]",
        "subject": "Welcome!",
        "html": "<h1>Hello!</h1>",
    },
)

data = res.json()

Contacts

SMTPfast supports Resend-style global contacts for subscriber state and personalization fields.

bash

curl -X POST https://smtpfa.st/api/v1/contacts \
  -H "Authorization: Bearer sf_live_your_api_key" \
  -H "Content-Type: application/json" \
  -d '{
    "email": "[email protected]",
    "firstName": "Steve",
    "lastName": "Wozniak",
    "unsubscribed": false,
    "properties": { "company": "Acme" }
  }'

Compatible Fields

Field	Type	Notes
from	string	Sender address (required)
to	string \| string[]	Accepts both formats
cc	string \| string[]	Accepts both formats
bcc	string \| string[]	Accepts both formats
reply_to	string \| string[]	Accepts both formats
subject	string	Email subject line
html	string	HTML body
text	string	Plain text fallback
tags	array	Array of { name, value } pairs
headers	object	Custom email headers
scheduled_at	string	ISO 8601 date, up to 30 days ahead

Key Differences

API key prefix

Resend keys start with re_. SMTPfast keys start with sf_live_ or sf_test_.

Base URL

Resend uses https://api.resend.com. SMTPfast uses https://smtpfa.st/api/v1.

Open and click tracking

Resend turns tracking on or off per domain. SMTPfast uses account settings, which a single email can override with track_clicks and track_opens, the matching X-SMTPfast-Track-* headers, or a per-link opt-out. Code written for Resend sends the same request and needs no change. See Tracking per email.

No SDK required

SMTPfast is a simple REST API. You do not need an SDK or client library. Just use fetch, axios, requests, or any HTTP client.