Email Clients & Servers

# Python Standard Library

Python has first-class email support baked in:

• smtplib — sending email via SMTP
• imaplib — reading and managing email via IMAP
• email — constructing and parsing messages (EmailMessage, email.parser)
• mailbox — reading local mailbox formats (Maildir, mbox)
• smtpd — deprecated mock SMTP server (removed in Python 3.12)
• socketserver — low-level TCP server, useful for rolling a custom mock SMTP

# Sending (smtplib)

import smtplib
from email.message import EmailMessage

msg = EmailMessage()
msg["From"] = "sender@example.com"
msg["To"] = "recipient@example.com"
msg["Subject"] = "Hello"
msg.set_content("Body text here")

with smtplib.SMTP("smtp.example.com", 587) as smtp:
    smtp.starttls()
    smtp.login("user", "password")
    smtp.send_message(msg)

# Reading (imaplib)

import imaplib, email

with imaplib.IMAP4_SSL("imap.example.com") as imap:
    imap.login("user", "password")
    imap.select("INBOX")
    _, ids = imap.search(None, "ALL")
    for uid in ids[0].split():
        _, data = imap.fetch(uid, "(RFC822)")
        msg = email.message_from_bytes(data[0][1])
        print(msg["subject"], msg["from"])

# Mock SMTP Server (aiosmtpd — recommended)

from aiosmtpd.controller import Controller
from aiosmtpd.handlers import AsyncMessage

class MyHandler(AsyncMessage):
    async def handle_message(self, message):
        print(f"Subject: {message['subject']}")

controller = Controller(MyHandler(), hostname="127.0.0.1", port=1025)
controller.start()

# JavaScript / TypeScript

JS/TS has no standard library for email. The language was born in the browser where direct TCP socket access is blocked. Node.js has low-level primitives (net, tls) but email was never added to core.

# What Node.js provides

# The ecosystem answer (npm)

# How Email Threading Works

Every email is an individual message. Threading is layered on top via headers:

Message-ID: <abc123@mail.example.com>       ← unique ID for this message
In-Reply-To: <xyz789@mail.example.com>      ← ID of the message being replied to
References: <xyz789@...> <abc123@...>       ← full chain of ancestor IDs

• The server stores flat, individual messages — no tree structure
• The client reconstructs threads by building a graph from these headers
• Fallback: if In-Reply-To is missing, clients match on Subject (stripping Re:, Fwd: etc.) — this is fuzzy and can cause incorrect grouping
• Gmail extension: adds a proprietary X-GM-THRID header so its IMAP server can return all messages in a thread directly, without client-side reconstruction

# Third-Party Libraries & Apps (Python)

# Libraries

# Local Mail Server Applications

# Mailpit

A lightweight, single-binary SMTP catcher for development and testing.

# What it is

• Accepts SMTP connections on localhost:1025
• Captures all mail — nothing is delivered unless explicitly configured
• Provides a web UI at localhost:8025 and a REST API for integration testing
• Also offers an optional POP3 server

# Sending / Relay Modes

Relay requires a config file:

host: smtp.gmail.com
port: 587
starttls: true
auth: plain
username: you@gmail.com
password: your-app-password

# What Mailpit Does NOT Support

• Multiple mailboxes — all captured mail lands in one shared inbox regardless of recipient. This was requested and closed as not planned. Workaround: run separate instances on different ports.
• Fetching external mail — Mailpit has no IMAP client. It cannot connect to Gmail or any external server. It only receives mail sent directly to it.

# Mailpit is not an email client

It cannot be used to read a real inbox (Gmail, Outlook, etc.). For that you need:

• A real IMAP client library (imaplib, imapflow)
• Or a desktop client (Thunderbird, Spark, etc.)
• Or a terminal client (aerc, neomutt)
• Or a sync tool (mbsync, offlineimap) to pull mail to a local Maildir