aiosmtplib

aiosmtplib is an asynchronous SMTP client built for Python’s asyncio. It mirrors the interface of the standard library’s smtplib but exposes every network operation as a coroutine, so applications can send email without blocking the event loop. A one-line aiosmtplib.send() helper covers the common case, while a full SMTP class gives fine-grained control over connections, commands, and sessions.

The library has zero runtime dependencies and supports the features expected of a modern SMTP client: STARTTLS and implicit TLS, SMTP authentication (LOGIN, PLAIN, CRAM-MD5), pipelining, and persistent connection reuse across multiple messages.

What You Get

• A one-line async aiosmtplib.send() helper for the common send-a-message case
• A full SMTP client class with explicit connect, login, and send-message coroutines
• STARTTLS and implicit TLS support for encrypted SMTP connections
• SMTP authentication mechanisms including LOGIN, PLAIN, and CRAM-MD5
• Zero runtime dependencies and full type hints

Common Use Cases

• Sending transactional email from an asyncio web application without blocking the loop
• Delivering notifications from an async background worker or task queue
• Reusing a single authenticated SMTP connection to send many messages efficiently
• Talking to a local or remote SMTP relay over STARTTLS with authentication

Under The Hood

Architecture — The source is cleanly split across focused modules in src/aiosmtplib: protocol.py implements the asyncio Protocol handling the wire connection, smtp.py and esmtp.py build the SMTP/ESMTP client on top, api.py exposes the high-level send() helper, and auth.py, email.py, response.py, and errors.py cover authentication, message flattening, response parsing, and the exception hierarchy. Coroutines drive every command exchange over the event loop.

Tech Stack — Pure Python (3.10+) with zero runtime dependencies, packaged via pyproject.toml. It ships type hints (typing.py, mypy.ini) and a full docs tree on Read the Docs; development uses pre-commit and CircleCI.

Code Quality — The test suite is thorough, with an in-repo test SMTP server (tests/smtpd.py) and dedicated suites for commands, auth methods, asyncio behaviour, and the API surface (test_commands, test_auth_methods, test_asyncio, test_api). mypy strict typing and pre-commit hooks enforce consistency; the module boundaries keep responsibilities small and readable.

API Design — The public API is deliberately smtplib-shaped, easing migration, while adding a single-call send() for the 90% case. Async/await is idiomatic throughout, keyword arguments for hostname/port/TLS/auth are discoverable, and the documented quickstart gets a message sent in a handful of lines.