Flaskframework~8 mins

Sending simple emails in Flask - Performance & Optimization

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Performance: Sending simple emails

MEDIUM IMPACT

This affects the server response time and user experience during email sending operations.

Sending an email when a user submits a form

Flask

import threading
from flask import Flask, request
from flask_mail import Mail, Message

app = Flask(__name__)
app.config.update(
    MAIL_SERVER='smtp.example.com',
    MAIL_PORT=587,
    MAIL_USE_TLS=True,
    MAIL_USERNAME='user@example.com',
    MAIL_PASSWORD='password'
)
mail = Mail(app)

def send_async_email(app, msg):
    with app.app_context():
        mail.send(msg)

@app.route('/send', methods=['POST'])
def send_email():
    msg = Message('Hello', sender='user@example.com', recipients=['recipient@example.com'])
    msg.body = 'This is a test email'
    thread = threading.Thread(target=send_async_email, args=(app, msg))
    thread.start()  # asynchronous call
    return 'Email sending started!'

Sending email in a separate thread frees the server to respond immediately, improving user experience.

📈 Performance GainNon-blocking response, reduces INP by hundreds of milliseconds to seconds.

Sending an email when a user submits a form

Flask

from flask import Flask, request
from flask_mail import Mail, Message

app = Flask(__name__)
app.config.update(
    MAIL_SERVER='smtp.example.com',
    MAIL_PORT=587,
    MAIL_USE_TLS=True,
    MAIL_USERNAME='user@example.com',
    MAIL_PASSWORD='password'
)
mail = Mail(app)

@app.route('/send', methods=['POST'])
def send_email():
    msg = Message('Hello', sender='user@example.com', recipients=['recipient@example.com'])
    msg.body = 'This is a test email'
    mail.send(msg)  # synchronous call
    return 'Email sent!'

The mail.send() call blocks the server until the email is sent, delaying the HTTP response and making the user wait.

📉 Performance CostBlocks server response for 500ms to several seconds depending on SMTP latency, increasing INP.

Performance Comparison

Pattern	Server Blocking	Response Delay	User Interaction Delay	Verdict
Synchronous email sending	Blocks server thread	Delays response by 500ms+	High INP, user waits	[X] Bad
Asynchronous email sending with threading	Non-blocking	Immediate response	Low INP, smooth UX	[OK] Good

Rendering Pipeline

Email sending in Flask affects server-side processing before response is sent. Synchronous sending blocks the server, delaying response and user interaction. Asynchronous sending offloads email sending to a background thread, allowing faster response.

→Server Processing

→Response Time

→User Interaction

⚠️ BottleneckServer Processing during synchronous email sending

Core Web Vital Affected

INP

This affects the server response time and user experience during email sending operations.

Optimization Tips

1Avoid synchronous email sending in request handlers to prevent blocking server response.

2Use background threads or task queues to send emails asynchronously.

3Monitor server response times in DevTools Network tab to detect blocking operations.

Performance Quiz - 3 Questions

Test your performance knowledge

What is the main performance problem with sending emails synchronously in a Flask route?

AIt causes layout shifts on the page

BIt increases CSS rendering time

CIt blocks the server response, increasing user wait time

DIt reduces the size of the email

DevTools: Network

How to check: Open DevTools, go to Network tab, submit the form that triggers email sending, and observe the response time of the request.

What to look for: Long response time indicates synchronous blocking; short response time with background email sending indicates good performance.