Email with attachments in Flask - Performance & Optimization

Performance: Email with attachments

MEDIUM IMPACT

This affects server response time and network load during email sending, impacting user wait time and server throughput.

Sending an email with attachments in a Flask app

Flask

from flask_mail import Mail, Message

mail = Mail(app)

msg = Message('Hello', sender='[e-mail]', recipients=['[e-mail]'])
with open('large_file.zip', 'rb') as f:
    # Ideally, implement streaming or chunked reading here
    msg.attach('large_file.zip', 'application/zip', f.read(1024*1024))  # read in chunks or stream

# Alternatively, send email asynchronously using background task
from threading import Thread

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

Thread(target=send_async_email, args=(app, msg)).start()

Reading in chunks or sending email asynchronously prevents blocking the main server thread, reducing memory spikes and improving user experience.

📈 Performance GainNon-blocking email send, reduces memory usage, improves server responsiveness

Sending an email with attachments in a Flask app

Flask

from flask_mail import Mail, Message

mail = Mail(app)

msg = Message('Hello', sender='[e-mail]', recipients=['[e-mail]'])
with open('large_file.zip', 'rb') as f:
    msg.attach('large_file.zip', 'application/zip', f.read())
mail.send(msg)

Reading the entire file into memory blocks the server and increases memory usage, causing slow response and possible crashes with large files.

📉 Performance CostBlocks server for duration of file read, high memory usage, increases response time by seconds for large files

Performance Comparison

Pattern	Memory Usage	Server Blocking	Network Load	Verdict
Read entire file synchronously	High (loads full file in memory)	Blocks server thread	Full file sent at once	[X] Bad
Read file in chunks or stream	Low (small buffer size)	Non-blocking if async	Full file sent efficiently	[OK] Good

Rendering Pipeline

Email sending with attachments involves server-side file reading and network transmission, which impacts server processing and network I/O rather than browser rendering.

→Server Processing

→Network Transmission

⚠️ BottleneckFile I/O and synchronous blocking during attachment reading

Optimization Tips

1Avoid reading large attachments fully into memory synchronously.

2Use asynchronous tasks to send emails to keep server responsive.

3Stream large files in chunks to reduce memory usage.

Performance Quiz - 3 Questions

What is a main performance issue when sending large email attachments synchronously in Flask?

A CSS styles cause layout shifts

B Browser rendering is delayed due to attachment size

C Server thread blocks during file read causing slow response

D JavaScript event handlers slow down

DevTools: Network panel (browser) and server logs

How to check: Use server logs or profiling tools to measure email send duration and memory usage; check network panel for upload size and timing if applicable

What to look for: Long server response times or high memory spikes indicate blocking; large payload size confirms attachment impact