Your app already knows how to send email. Let it.

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