Send email with Flask

Prerequisites

Python 3.8 or later
A Flask 2 or Flask 3 project
Authorize your sending domain
Create or retrieve your SendLayer API key

Installation

Install the SDK along with Flask and python-dotenv.

pip install flask sendlayer python-dotenv

Storing your API key

Create a .env file in your project root.

.env

SENDLAYER_API_KEY=your-api-key
FLASK_APP=app

Add .env to .gitignore so the key stays out of version control.

Creating the email client

Initialize the client once when the application starts and share it across routes. A module-level instance avoids rebuilding the client on every request.

app/email_client.py

import os

from dotenv import load_dotenv
from sendlayer import SendLayer

load_dotenv()

api_key = os.environ.get("SENDLAYER_API_KEY")

if not api_key:
    raise RuntimeError("SENDLAYER_API_KEY is not set")

sendlayer = SendLayer(api_key)

The startup check fails fast on a missing key. Without it, the first request becomes the place the problem surfaces.

Sending an email from a route

Add a POST route that reads JSON and calls Emails.send().

app/__init__.py

from flask import Flask, jsonify, request

from app.email_client import sendlayer


def create_app():
    app = Flask(__name__)

    @app.post("/api/send-email")
    def send_email():
        data = request.get_json()

        response = sendlayer.Emails.send(
            sender={"email": "sender@example.com", "name": "Acme Support"},
            to=[{"email": "support@example.com", "name": "Support Team"}],
            reply_to=[{"email": data["email"], "name": data["name"]}],
            subject=f"New message from {data['name']}",
            text=data["message"],
        )

        return jsonify({"message_id": response}), 200

    return app

The reply_to field points at the person who filled in the form, so support agents can reply from their inbox.

Use a sender address on a domain you authorized in SendLayer. Requests from an unauthorized domain fail.

Run the application:

flask run

Organizing routes with a blueprint

A blueprint keeps email routes separate from the rest of the application. This structure scales better once you add more endpoints.

app/routes/email.py

from flask import Blueprint, jsonify, request

from app.email_client import sendlayer

email_bp = Blueprint("email", __name__, url_prefix="/api")


@email_bp.post("/send-email")
def send_email():
    data = request.get_json(silent=True) or {}

    missing = [field for field in ("email", "subject", "message") if not data.get(field)]
    if missing:
        return jsonify({"error": f"Missing required fields: {', '.join(missing)}"}), 400

    response = sendlayer.Emails.send(
        sender="sender@example.com",
        to=data["email"],
        subject=data["subject"],
        text=data["message"],
    )

    return jsonify({"message_id": response})

app/__init__.py

from flask import Flask

from app.routes.email import email_bp


def create_app():
    app = Flask(__name__)
    app.register_blueprint(email_bp)
    return app

Validating the payload first saves a round trip to the API and returns a clearer error to the caller.

Sending HTML emails

Add the html parameter for rich content. Keep text as a fallback for clients that block HTML.

app/routes/welcome.py

from flask import Blueprint, jsonify, request

from app.email_client import sendlayer

welcome_bp = Blueprint("welcome", __name__, url_prefix="/api")


@welcome_bp.post("/welcome")
def send_welcome_email():
    data = request.get_json()

    response = sendlayer.Emails.send(
        sender={"email": "hello@example.com", "name": "Acme"},
        to=data["email"],
        subject="Welcome to Acme",
        html=(
            "<html><body>"
            "<h1>Welcome aboard</h1>"
            "<p>Your account is ready. "
            "<a href='https://example.com/login'>Sign in</a> to get started.</p>"
            "</body></html>"
        ),
        text="Your account is ready. Sign in at https://example.com/login to get started.",
        tags=["welcome-email"],
    )

    return jsonify({"message_id": response})

The tags list labels the message. Tags appear in your event history, which helps you separate welcome emails from other traffic.

Rendering emails with Jinja templates

Flask already renders Jinja templates, so you can reuse that engine for email bodies. Place the template under templates/emails/.

templates/emails/welcome.html

<html>
  <body>
    <h1>Welcome, {{ name }}</h1>
    <p>Your {{ plan }} plan is active.</p>
    <p><a href="{{ login_url }}">Sign in to your account</a></p>
  </body>
</html>

Render the template, then pass the result as the html parameter.

app/routes/welcome.py

from flask import Blueprint, jsonify, render_template, request

from app.email_client import sendlayer

welcome_bp = Blueprint("welcome", __name__, url_prefix="/api")


@welcome_bp.post("/welcome")
def send_welcome_email():
    data = request.get_json()

    html_body = render_template(
        "emails/welcome.html",
        name=data["name"],
        plan=data.get("plan", "Starter"),
        login_url="https://example.com/login",
    )

    response = sendlayer.Emails.send(
        sender={"email": "hello@example.com", "name": "Acme"},
        to=data["email"],
        subject=f"Welcome, {data['name']}",
        html=html_body,
        text=f"Welcome, {data['name']}. Sign in at https://example.com/login.",
    )

    return jsonify({"message_id": response})

Keep email templates in their own folder. Email clients support a narrow subset of CSS, so those files rarely share styles with your web pages.

Testing the endpoint

Start the development server.

flask run

Then post a request from another terminal:

curl --request POST \
  --url http://localhost:5000/api/send-email \
  --header 'Content-Type: application/json' \
  --data '{
  "email": "paulie@example.com",
  "subject": "Testing Flask",
  "message": "Testing the send endpoint."
}'

A successful request returns the message ID:

{
  "message_id": "06e4491f-fc5a-49cb-bc57-xxxxxx"
}

Open the Email logs page in your SendLayer dashboard to confirm delivery.

Next steps

Python SDK Reference

Review every method and parameter in the Python SDK.

Managing Webhooks

Create, list, and delete webhook subscriptions.

Retrieving Events

Query delivery events for any message.

Rate Limits

Understand recipient and request limits.

Getting Started

SDKs

Guides

Integrations

Releases

Prerequisites

Installation

Storing your API key

Creating the email client

Sending an email from a route

Organizing routes with a blueprint

Sending HTML emails

Rendering emails with Jinja templates

Testing the endpoint

Next steps

Python SDK Reference

Managing Webhooks

Retrieving Events

Rate Limits

​Prerequisites

​Installation

​Storing your API key

​Creating the email client

​Sending an email from a route

​Organizing routes with a blueprint

​Sending HTML emails

​Rendering emails with Jinja templates

​Testing the endpoint

​Next steps

Python SDK Reference

Managing Webhooks

Retrieving Events

Rate Limits

Prerequisites

Installation

Storing your API key

Creating the email client

Sending an email from a route

Organizing routes with a blueprint

Sending HTML emails

Rendering emails with Jinja templates

Testing the endpoint

Next steps