Flask + MailKite

Flask has no swappable mail-transport interface — Flask-Mail wraps smtplib directly. That means there's no MailKite package to install: point Flask-Mail's existing config at MailKite's SMTP relay and every mail.send() call goes through, with the same DKIM-signed delivery and dashboard logs as the API.

What you need

• A verified domain with SPF + DKIM published
• Your API key (mk_live_…) — it doubles as the SMTP password
• Flask 2+ and Flask-Mail

A runnable starter

Rather than a snippet, MailKite ships a full working Flask app: a send-mail form (SMTP relay + API mode side by side), a README, and env-var docs — starters/flask/ in the MailKite repo. This page walks through the same pieces.

Configure Flask-Mail

Set your env vars, then point Flask-Mail's config at MailKite's relay. These are the real connection details — see the SMTP relay reference for the full spec.

# .env
MAILKITE_API_KEY=mk_live_...
MAIL_DEFAULT_SENDER=hello@yourdomain.com
FLASK_SECRET_KEY=change-me

# app.py
import os
from flask import Flask
from flask_mail import Mail

app = Flask(__name__)
app.config["SECRET_KEY"] = os.environ["FLASK_SECRET_KEY"]

# Flask-Mail wraps smtplib directly — no adapter needed, just config.
app.config["MAIL_SERVER"] = "smtp.mailkite.dev"
app.config["MAIL_PORT"] = 587          # STARTTLS (465 = implicit TLS also works)
app.config["MAIL_USE_TLS"] = True
app.config["MAIL_USERNAME"] = "mailkite"   # literal string — MailKite only checks the password
app.config["MAIL_PASSWORD"] = os.environ["MAILKITE_API_KEY"]  # the password IS your API key
app.config["MAIL_DEFAULT_SENDER"] = os.environ["MAIL_DEFAULT_SENDER"]

mail = Mail(app)

Send email

Use Flask-Mail's normal Message + mail.send() — nothing MailKite-specific:

# app.py — a route that sends mail
from flask import request, flash, redirect, url_for
from flask_mail import Message

@app.post("/send")
def send():
    msg = Message(
        subject=request.form["subject"],
        recipients=[request.form["to"]],
        html=request.form["body"],
    )
    mail.send(msg)
    flash("Sent!", "success")
    return redirect(url_for("index"))

API mode (alternative)

Once you need something SMTP can't express — templates, batch sends, scheduled sends — call the MailKite Python SDK directly instead. Same MAILKITE_API_KEY, a different transport:

# mailkite_api.py — API mode alternative (no SMTP)
# pip install mailkite-dev
import os
from mailkite import MailKite

mk = MailKite(os.environ["MAILKITE_API_KEY"])

def send_via_api(to, subject, body, sender):
    return mk.send({
        "from": sender,
        "to": to,
        "subject": subject,
        "html": body,
    })

Test it

# Verify the app boots and the form renders
python -m py_compile app.py mailkite_api.py
flask --app app run --debug
curl -s -o /dev/null -w "%{http_code}\n" http://127.0.0.1:5000/

Troubleshooting

• smtplib.SMTPAuthenticationError — MAIL_PASSWORD must be your mk_live_… API key, not a separate SMTP credential.
• Connection refused — check MAIL_PORT; MailKite's relay listens on 587 (STARTTLS) or 465 (implicit TLS), never 25.
• 403 on send — the From: address isn't on a verified domain yet.
• API-mode call raises something other than a MailKite error — wrap SDK calls in a broad except Exception in production code; transport-layer failures (timeouts, proxies, WAFs) don't always surface as a clean SDK exception.

See the SMTP relay docs for the full connection reference, or Send API for the Python SDK's complete surface (templates, batch sends, attachments).