Send email with Python (smtplib) over SMTP

# standard library only — nothing to install

import os, smtplib, ssl
from email.message import EmailMessage

HOST = os.environ.get("SMTP_HOST", "smtp.queensmtp.com")
PORT = int(os.environ.get("SMTP_PORT", "587"))
USER = os.environ["SMTP_USER"]
PASS = os.environ["SMTP_PASS"]

def send(to: str, subject: str, text: str, html: str | None = None, attachment: str | None = None) -> None:
    msg = EmailMessage()
    msg["From"] = "Your App <hello@yourdomain.com>"   # verified sending domain
    msg["To"] = to
    msg["Subject"] = subject
    msg.set_content(text)
    if html:
        msg.add_alternative(html, subtype="html")
    if attachment:
        with open(attachment, "rb") as f:
            msg.add_attachment(f.read(), maintype="application", subtype="pdf",
                               filename=os.path.basename(attachment))

    ctx = ssl.create_default_context()
    if PORT == 465:                                   # implicit TLS
        with smtplib.SMTP_SSL(HOST, PORT, context=ctx, timeout=20) as s:
            s.login(USER, PASS)
            s.send_message(msg)
    else:                                             # STARTTLS on 587 / 2525
        with smtplib.SMTP(HOST, PORT, timeout=20) as s:
            s.ehlo()
            s.starttls(context=ctx)
            s.login(USER, PASS)
            s.send_message(msg)

if __name__ == "__main__":
    send("customer@example.com", "Your receipt", "Thanks for your order #1042",
         "<h1>Thanks for your order</h1><p>#1042 is confirmed.</p>")