Send email with Resend
Transactional email is the mail your app sends one recipient at a time — receipts, password resets, magic links, notifications. This guide sends one through Resend, over both the API and SMTP, then shows the same send on MailKite.
What you'll need
- A domain you control, with access to its DNS records.
- An API key from your Resend account — the same key works for the API and SMTP.
Part 1 — Send with Resend
1. Authenticate your domain
So your mail passes authentication and lands in the inbox, add your domain
under Domains in Resend and publish the records it shows you. Resend
puts sending records on a send subdomain — an MX for
bounce and complaint feedback, an SPF TXT, and a DKIM
TXT:
Type Name Value Priority
MX send.yourdomain.com feedback-smtp.us-east-1.amazonses.com 10
TXT send.yourdomain.com v=spf1 include:amazonses.com ~all
TXT resend._domainkey.yourdomain.com p=… (DKIM public key from your dashboard) Resend shows the exact per-domain DKIM value in the dashboard — copy it from there.
2. Send over the API
POST to Resend's /emails endpoint with your API key.
The from address must be on the domain you just authenticated:
curl -X POST https://api.resend.com/emails \
-H "Authorization: Bearer $RESEND_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"from": "MyApp <hello@yourdomain.com>",
"to": ["ada@example.com"],
"subject": "Your invoice #1042",
"html": "<p>Thanks! Your receipt is attached.</p>"
}' ← 200 OK
{ "id": "49a3999c-0ce1-4ea6-ab68-afcd6dc2e794" } Or with the SDK — same fields, in your language:
import { Resend } from "resend";
const resend = new Resend(process.env.RESEND_API_KEY);
const { data, error } = await resend.emails.send({
from: "MyApp <hello@yourdomain.com>",
to: ["ada@example.com"],
subject: "Your invoice #1042",
html: "<p>Thanks! Your receipt is attached.</p>",
}); 3. Or send over SMTP
Prefer SMTP? Point your app at Resend's relay. The username is
resend and the password is your API key:
Host: smtp.resend.com
Port: 587 # STARTTLS (also 465/2465 SMTPS, 25/2587)
Username: resend
Password: re_... # your API key
From: hello@yourdomain.com That's a transactional send on Resend: authenticate the domain, then call the API or the SMTP relay.
Part 2 — The same, on MailKite
When we built sending into MailKite, we wanted three things to be true: one set of DNS records, one API key that works for both HTTP and SMTP, and a body you don't have to nest. Here's the same message.
1. Authenticate your domain
Publish three TXT records — SPF, a MailKite DKIM key, and DMARC.
The same domain also receives mail once its MX points at MailKite:
Type Name Value
TXT yourdomain.com v=spf1 include:mailkite.dev ~all
TXT mailkite._domainkey.yourdomain.com v=DKIM1; k=rsa; p=…
TXT _dmarc.yourdomain.com v=DMARC1; p=none; The exact DKIM value is shown in the dashboard; MailKite verifies the records for you over DNS.
2. Send over the API
One POST /v1/send with a flat from/to/
subject/html body. A 202 means it's
queued:
Pick your language in the card — the dashboard and the AI assistant do the same send, and every SDK takes the same fields:
Messages → Compose
From hello@myapp.ai
To ada@example.com
Subject Your invoice #1042
Body Thanks! Receipt attached.
Click [ Send ]Email ada@example.com from hello@myapp.ai
with subject "Your invoice #1042" and body "Thanks! Receipt attached."import { MailKite } from "mailkite";
const mk = new MailKite(process.env.MAILKITE_API_KEY);
const { id, status } = await mk.send({
from: "hello@myapp.ai",
to: "ada@example.com",
subject: "Your invoice #1042",
html: "<p>Thanks! Receipt attached.</p>",
});import os
from mailkite import MailKite
mk = MailKite(os.environ["MAILKITE_API_KEY"])
res = mk.send({
"from": "hello@myapp.ai",
"to": "ada@example.com",
"subject": "Your invoice #1042",
"html": "<p>Thanks! Receipt attached.</p>",
})<?php
$mk = new \MailKite\Client(getenv('MAILKITE_API_KEY'));
$res = $mk->send([
'from' => 'hello@myapp.ai',
'to' => 'ada@example.com',
'subject' => 'Your invoice #1042',
'html' => '<p>Thanks! Receipt attached.</p>',
]);MailKite mk = new MailKite(System.getenv("MAILKITE_API_KEY"));
Object res = mk.send(Map.of(
"from", "hello@myapp.ai",
"to", "ada@example.com",
"subject", "Your invoice #1042",
"html", "<p>Thanks! Receipt attached.</p>"
));mk := mailkite.New(os.Getenv("MAILKITE_API_KEY"))
res, err := mk.Send(mailkite.Message{
From: "hello@myapp.ai",
To: "ada@example.com",
Subject: "Your invoice #1042",
HTML: "<p>Thanks! Receipt attached.</p>",
})require "mailkite"
mk = Mailkite::Client.new(ENV["MAILKITE_API_KEY"])
res = mk.send(
"from" => "hello@myapp.ai",
"to" => "ada@example.com",
"subject" => "Your invoice #1042",
"html" => "<p>Thanks! Receipt attached.</p>"
)curl https://api.mailkite.dev/v1/send \
-H "Authorization: Bearer $MAILKITE_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"from": "hello@myapp.ai",
"to": "ada@example.com",
"subject": "Your invoice #1042",
"html": "<p>Thanks! Receipt attached.</p>"
}' Messages → Sent
● queued msg_2Hk9… ada@example.com Your invoice #1042Sent — message msg_2Hk9…, status queued.← 202 Accepted
{ "id": "msg_2Hk9…", "status": "queued" }← 202 Accepted
{ "id": "msg_2Hk9…", "status": "queued" }← 202 Accepted
{ "id": "msg_2Hk9…", "status": "queued" }← 202 Accepted
{ "id": "msg_2Hk9…", "status": "queued" }← 202 Accepted
{ "id": "msg_2Hk9…", "status": "queued" }← 202 Accepted
{ "id": "msg_2Hk9…", "status": "queued" }← 202 Accepted
{ "id": "msg_2Hk9…", "status": "queued" } 3. Or send over SMTP
The same key is your SMTP password — no separate credential to generate:
import nodemailer from "nodemailer";
const tx = nodemailer.createTransport({
host: "smtp.mailkite.dev",
port: 587, // STARTTLS · use 465 for implicit TLS
auth: {
user: "mailkite", // any username works
pass: process.env.MAILKITE_API_KEY, // your mk_live_… key
},
});
await tx.sendMail({
from: "hello@myapp.ai", // an address on a verified domain
to: "ada@example.com",
subject: "Your invoice #1042",
html: "<p>Thanks! Receipt attached.</p>",
});import os, smtplib
from email.message import EmailMessage
msg = EmailMessage()
msg["From"] = "hello@myapp.ai" # an address on a verified domain
msg["To"] = "ada@example.com"
msg["Subject"] = "Your invoice #1042"
msg.set_content("Thanks! Receipt attached.")
# Standard library — no dependency to install.
with smtplib.SMTP("smtp.mailkite.dev", 587) as s:
s.starttls()
s.login("mailkite", os.environ["MAILKITE_API_KEY"]) # password = your key
s.send_message(msg)<?php
use PHPMailer\PHPMailer\PHPMailer;
$mail = new PHPMailer(true);
$mail->isSMTP();
$mail->Host = 'smtp.mailkite.dev';
$mail->SMTPAuth = true;
$mail->Username = 'mailkite'; // any username
$mail->Password = getenv('MAILKITE_API_KEY'); // your mk_live_… key
$mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;
$mail->Port = 587; // or 465 for implicit TLS
$mail->setFrom('hello@myapp.ai'); // an address on a verified domain
$mail->addAddress('ada@example.com');
$mail->Subject = 'Your invoice #1042';
$mail->Body = 'Thanks! Receipt attached.';
$mail->send();Properties p = new Properties();
p.put("mail.smtp.host", "smtp.mailkite.dev");
p.put("mail.smtp.port", "587"); // 465 for implicit TLS
p.put("mail.smtp.auth", "true");
p.put("mail.smtp.starttls.enable", "true");
Session s = Session.getInstance(p, new Authenticator() {
protected PasswordAuthentication getPasswordAuthentication() {
// username = anything · password = your mk_live_… key
return new PasswordAuthentication("mailkite", System.getenv("MAILKITE_API_KEY"));
}
});
Message msg = new MimeMessage(s);
msg.setFrom(new InternetAddress("hello@myapp.ai")); // a verified domain
msg.setRecipients(Message.RecipientType.TO, InternetAddress.parse("ada@example.com"));
msg.setSubject("Your invoice #1042");
msg.setText("Thanks! Receipt attached.");
Transport.send(msg);auth := smtp.PlainAuth("", "mailkite", os.Getenv("MAILKITE_API_KEY"), "smtp.mailkite.dev")
msg := []byte("From: hello@myapp.ai\r\n" + // an address on a verified domain
"To: ada@example.com\r\n" +
"Subject: Your invoice #1042\r\n\r\n" +
"Thanks! Receipt attached.\r\n")
// STARTTLS on 587; SendMail upgrades the connection before AUTH.
err := smtp.SendMail("smtp.mailkite.dev:587", auth,
"hello@myapp.ai", []string{"ada@example.com"}, msg)require "mail"
Mail.defaults do
delivery_method :smtp, {
address: "smtp.mailkite.dev",
port: 587, # 465 for implicit TLS
user_name: "mailkite", # any username
password: ENV["MAILKITE_API_KEY"], # your mk_live_… key
enable_starttls_auto: true,
}
end
Mail.deliver do
from "hello@myapp.ai" # an address on a verified domain
to "ada@example.com"
subject "Your invoice #1042"
body "Thanks! Receipt attached."
end See Send API and SMTP relay for the full reference — attachments, headers, and delivery events.
A little about MailKite
MailKite is programmable email for developers and AI agents. One API key and
one set of DNS records cover both directions: send with a single
POST (or the SMTP relay), and the same verified domain receives
replies as a signed webhook of clean JSON.
Behind the send, MailKite routes across multiple providers to keep you in the
inbox — and any address can be handed to an agent
that reads and replies on its own. Sending and receiving are on the
free tier.
Next: the Send API for attachments and delivery tracking, or the Quickstart to get a verified domain and API key.