📨 dsmtpd — A Lightweight SMTP Server for Modern Developers

As developers, we often need to test email delivery locally—without setting up a full-blown mail server. That’s exactly where dsmtpd comes in: a tiny, practical SMTP daemon written in Python, designed to receive emails and store them directly in a local Maildir folder.

🚀 What is dsmtpd?

dsmtpd (Developer SMTP Daemon) is an asynchronous mini SMTP server based on aiosmtpd. It listens on a network interface (by default 127.0.0.1:1025) and saves all incoming emails into a Maildir-compatible directory.

It’s perfect for:

  • Developers testing email features locally
  • Debugging and inspection of email content
  • Integration tests or CI pipelines needing a fake SMTP backend

⚙️ Key Features

  • 📬 Async SMTP server using asyncio and aiosmtpd
  • 🗂️ Saves emails into Maildir format
  • 🔐 Proper locking/unlocking of the mailbox
  • 📄 Intelligent logging of received emails (sender, recipients, subject)
  • 🧰 CLI arguments for port/interface/maildir configuration and size limit

📦 Quick Example

$ python -m dsmtpd -i 127.0.0.1 -p 1025 -d ./maildir

In your Python application

import smtplib
from email.message import EmailMessage

msg = EmailMessage()
msg.set_content("Hello from dsmtpd")
msg["Subject"] = "Test"
msg["From"] = "me@example.com"
msg["To"] = "you@example.com"

with smtplib.SMTP("localhost", 1025) as s:
    s.send_message(msg)

🎉 The message will be saved to ./maildir — you can explore it with any Maildir-compatible reader or script.

👨‍💻 Why I Built It

I originally created dsmtpd because I needed a simple way to test outgoing emails without sending anything to the real world. By combining the power of aiosmtpd with a local Maildir backend, I ended up with a reliable little SMTP daemon that still helps me today in my daily development and testing tasks.

📚 Useful Links

• 🔗 GitHub Repository : https://github.com/matrixise/dsmtpd • 📄 License: BSD