Transactional Email with Resend

Every real app needs to send email — welcome messages, password resets, order confirmations. This tutorial shows how to integrate Resend into Flask cleanly, keeping your routes thin and email logic in a reusable helper.

Why Resend?

Resend is a modern email API built for developers:

Simple Python SDK — one function call to send
Reliable delivery with SPF/DKIM handled automatically
Free tier — 3,000 emails/month, 100/day
Dashboard with delivery logs and per-email analytics

No SMTP port confusion, no TLS certificate setup, no MX records to manage yourself.

Setup

Install the SDK:

pip install resend

Add your API key to .env:

RESEND_API_KEY=re_your_key_here
[email protected]

Get your API key at resend.com → API Keys → Create API Key.

Create the Email Helper

Create utils/email.py:

import os
import resend
from flask import render_template_string

resend.api_key = os.environ.get("RESEND_API_KEY")
FROM_EMAIL = os.environ.get("FROM_EMAIL", "[email protected]")

def send_email(to, subject, html):
    """Send a single email. Returns True on success, False on failure."""
    try:
        resend.Emails.send({
            "from": FROM_EMAIL,
            "to": to,
            "subject": subject,
            "html": html
        })
        return True
    except Exception as e:
        print(f"Email error: {e}")
        return False

def send_welcome_email(user_email, user_name):
    """Send a welcome email to a new user."""
    html = render_template_string("""
        <h1>Welcome, {{ name }}!</h1>
        <p>Thanks for joining. Your account is ready.</p>
        <p>— The Team</p>
    """, name=user_name)

    return send_email(
        to=user_email,
        subject=f"Welcome, {user_name}!",
        html=html
    )

Key design decisions:

send_email() is generic — accepts any HTML string
Specific helpers like send_welcome_email() build on top of it
Returns True/False so routes can handle failures without crashing
The API key is loaded once at module import time

Calling It From a Flask Route

from utils.email import send_welcome_email

@app.route("/register", methods=["POST"])
def register():
    name = request.form.get("name")
    email = request.form.get("email")
    password = request.form.get("password")

    hashed = generate_password_hash(password)
    with get_db() as conn:
        with conn.cursor() as cur:
            cur.execute(
                "INSERT INTO users (name, email, password_hash) VALUES (%s, %s, %s)",
                (name, email, hashed)
            )
            conn.commit()

    # Send welcome email — failure does not crash the request
    send_welcome_email(email, name)

    return redirect("/dashboard")

The route stays thin. Email logic lives entirely in utils/email.py.

Richer HTML Email Template

For fully branded emails, define the HTML as a template string:

WELCOME_TEMPLATE = """
<!DOCTYPE html>
<html>
<body style="font-family: sans-serif; max-width: 600px; margin: 0 auto; padding: 20px;">
    <h2 style="color: #333;">Welcome, {{ name }}!</h2>
    <p>Your account has been created successfully.</p>
    <a href="{{ app_url }}/dashboard"
       style="background: #333; color: white; padding: 12px 24px;
              text-decoration: none; border-radius: 4px; display: inline-block;">
        Go to Dashboard
    </a>
    <p style="color: #999; font-size: 12px; margin-top: 32px;">
        You received this because you signed up at {{ app_url }}.
    </p>
</body>
</html>
"""

def send_welcome_email(user_email, user_name):
    html = render_template_string(
        WELCOME_TEMPLATE,
        name=user_name,
        app_url=os.environ.get("APP_URL", "https://yourapp.com")
    )
    return send_email(user_email, f"Welcome, {user_name}!", html)

Skip Real Sends in Development

During development, log the email instead of sending it:

def send_email(to, subject, html):
    if os.environ.get("FLASK_ENV") == "development":
        print(f"[DEV EMAIL] To: {to} | Subject: {subject}")
        return True
    try:
        resend.Emails.send({
            "from": FROM_EMAIL,
            "to": to,
            "subject": subject,
            "html": html
        })
        return True
    except Exception as e:
        print(f"Email error: {e}")
        return False

Add FLASK_ENV=development to your .env to skip real sends locally.

AI Prompt That Generated This

"Create utils/email.py for a Flask app using the Resend Python SDK. Include a generic send_email(to, subject, html) function that returns True/False and a send_welcome_email(email, name) helper. Load RESEND_API_KEY from os.environ. Handle exceptions without crashing the caller."

Next Steps

Build Password Reset Flow using this helper for reset link emails
Scale sends with Background Tasks with Celery for bulk operations

Transactional Email with Resend

Transactional Email with Resend

Why Resend?

Setup

Create the Email Helper

Calling It From a Flask Route

Richer HTML Email Template

Skip Real Sends in Development

AI Prompt That Generated This

Next Steps

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