Transactional mail, over plain SMTP.

# injected into your app on the next deploy
SMTP_HOST=smtp.…
SMTP_PORT=587
SMTP_USER=…
SMTP_PASSWORD=•••
SMTP_FROM=hello@example.com
SMTP_URL=smtp://…

import nodemailer from "nodemailer";

const mail = nodemailer.createTransport(process.env.SMTP_URL);
await mail.sendMail({
  from: process.env.SMTP_FROM,
  to: "customer@example.org",
  subject: "Your receipt",
  text: "Thanks for your order.",
});

import os, smtplib
from email.message import EmailMessage

msg = EmailMessage()
msg["From"], msg["To"] = os.environ["SMTP_FROM"], "customer@example.org"
msg["Subject"] = "Your receipt"
with smtplib.SMTP(os.environ["SMTP_HOST"], 587) as s:
    s.starttls(); s.login(os.environ["SMTP_USER"], os.environ["SMTP_PASSWORD"]); s.send_message(msg)