Email Automation: smtplib & MIME

By the end of this lesson you can:

• Build an email with EmailMessage (from, to, subject, body).
• Connect to an SMTP server securely and authenticate.
• Use an app password from the environment, not your real password.
• Send to multiple recipients and handle send failures gracefully.

Sending mail has two parts: compose the message (headers + body, the MIME format) and transmit it through an SMTP server (the postal service). Python gives you EmailMessage for the first and smtplib for the second.

Gmail:    smtp.gmail.com        port 587 (STARTTLS) or 465 (SSL)
Outlook:  smtp.office365.com    port 587
Most:     587 + STARTTLS is the modern default

Build the message

from email.message import EmailMessage

msg = EmailMessage()
msg["From"] = "bot@example.com"
msg["To"] = "boss@example.com"
msg["Subject"] = "Daily report — 2026-05-28"
msg.set_content("Hi,\n\nToday's numbers are attached.\n\n— The Bot")

EmailMessage handles MIME for you. Set headers like dict keys; set_content sets the plain-text body. (HTML and attachments come next lesson.)

Send over TLS

import os, smtplib
from dotenv import load_dotenv

load_dotenv()
SMTP_HOST = "smtp.gmail.com"
SMTP_PORT = 587
USER = os.environ["SMTP_USER"]          # your email address
PASS = os.environ["SMTP_PASS"]          # an APP PASSWORD, from .env

with smtplib.SMTP(SMTP_HOST, SMTP_PORT) as server:
    server.starttls()                   # upgrade the connection to encrypted
    server.login(USER, PASS)
    server.send_message(msg)
print("sent ✅")

• SMTP(...) in a with block closes the connection automatically.
• starttls() encrypts the connection — always call it before login, or your password travels in plaintext.
• send_message(msg) transmits the EmailMessage.

App passwords, not real passwords

SSL alternative (port 465)

import smtplib, ssl

context = ssl.create_default_context()
with smtplib.SMTP_SSL(SMTP_HOST, 465, context=context) as server:
    server.login(USER, PASS)
    server.send_message(msg)

SMTP_SSL is encrypted from the first byte (no starttls needed). Use whichever port your provider prefers — 587+STARTTLS is the common default.

Multiple recipients

msg["To"] = "a@example.com, b@example.com"     # comma-separated
msg["Cc"] = "manager@example.com"
msg["Bcc"] = "archive@example.com"             # hidden recipients

send_message reads To/Cc/Bcc automatically. Use Bcc for bulk so recipients don't see each other's addresses.

Goal: a small send_email() function you can call from any automation, with config from the environment, TLS, logging, and graceful error handling.

import os, smtplib, logging
from email.message import EmailMessage
from dotenv import load_dotenv

load_dotenv()
logging.basicConfig(level=logging.INFO, format="%(levelname)s %(message)s")
log = logging.getLogger("mailer")

def send_email(to: list[str], subject: str, body: str,
               cc: list[str] | None = None) -> bool:
    host = os.getenv("SMTP_HOST", "smtp.gmail.com")
    port = int(os.getenv("SMTP_PORT", "587"))
    user = os.environ["SMTP_USER"]
    password = os.environ["SMTP_PASS"]

    msg = EmailMessage()
    msg["From"] = user
    msg["To"] = ", ".join(to)
    if cc:
        msg["Cc"] = ", ".join(cc)
    msg["Subject"] = subject
    msg.set_content(body)

    try:
        with smtplib.SMTP(host, port, timeout=30) as server:
            server.starttls()
            server.login(user, password)
            server.send_message(msg)
        log.info("sent '%s' to %d recipient(s)", subject, len(to))
        return True
    except smtplib.SMTPAuthenticationError:
        log.error("auth failed — check SMTP_USER and app password SMTP_PASS")
    except smtplib.SMTPException as e:
        log.error("send failed: %s", e)
    return False

send_email(
    to=["boss@example.com"],
    subject="Daily report — 2026-05-28",
    body="Hi,\n\nReport summary: 412 orders, total 84,210.50.\n\n— The Bot",
)

INFO sent 'Daily report — 2026-05-28' to 1 recipient(s)

Read the code

Everything risky is handled: config and the app password come from the environment, the connection is TLS-encrypted, and the two most common failures — bad auth and general send errors — are caught with actionable messages rather than a raw traceback. The function returns a bool so a calling pipeline can decide what to do if delivery fails. Drop this into the report generator (Lesson 22) and your reports email themselves; next lesson we attach the actual files.

Use the local debug server (above) or Mailtrap so you don't send real mail while learning.

Sending email = compose with EmailMessage (From/To/Subject headers, set_content for the body) and transmit with smtplib. Connect with SMTP(host, 587) + starttls() (or SMTP_SSL on 465), then login and send_message inside a with block. Authenticate with a revocable app password from the environment — never your real password, never in code. Catch SMTPAuthenticationError and SMTPException for clear failures, send to lists individually for robustness, and test against a debug/Mailtrap server so you never accidentally spam.