aiosmtpd - An asyncio based SMTP server

Project License on GitHub Travis CI Build Status Code Coverage Semmle/ quality Documentation Status PyPI Package

Discourse status

The Python standard library includes a basic SMTP server in the smtpd module, based on the old asynchronous libraries asyncore and asynchat. These modules are quite old and are definitely showing their age. asyncore and asynchat are difficult APIs to work with, understand, extend, and fix.

With the introduction of the asyncio module in Python 3.4, a much better way of doing asynchronous I/O is now available. It seems obvious that an asyncio-based version of the SMTP and related protocols are needed for Python 3. This project brings together several highly experienced Python developers collaborating on this reimplementation.

This package provides such an implementation of both the SMTP and LMTP protocols.


You need at least Python 3.6 to use this library. Both Windows and *nix are supported.


aiosmtpd is released under the Apache License version 2.0.

Project details

As of 2016-07-14, aiosmtpd has been put under the aio-libs umbrella project and moved to GitHub.

The best way to contact the developers is through the GitHub links above. You can also request help by submitting a question on StackOverflow.


You can install this package in a virtual environment like so:

$ python3 -m venv /path/to/venv
$ source /path/to/venv/bin/activate
$ python install

This will give you a command line script called smtpd which implements the SMTP server. Use smtpd --help for details.

You will also have access to the aiosmtpd library, which you can use as a testing environment for your SMTP clients. See the documentation links above for details.


You’ll need the tox tool to run the test suite for Python 3. Once you’ve got that, run:

$ tox

Individual tests can be run like this:

$ tox -e py36-nocov -- -P <pattern>

where <pattern> is a Python regular expression matching a test name.

You can also add the -E option to boost debugging output, e.g.:

$ tox -e py36-nocov -- -E

and these options can be combined:

$ tox -e py36-nocov -- -P test_connection_reset_during_DATA -E

Supported ‘testenvs’

In general, the -e parameter to tox specifies one (or more) testenv to run (separate using comma if more than one testenv). The following testenvs have been configured and tested:

  • {py36,py37,py38,py39,pypy3}-{nocov,cov,diffcov,profile}

    Specifies the interpreter to run and the kind of testing to perform.

    • nocov = no coverage testing. Tests will run verbosely.
    • cov = with coverage testing. Tests will run in brief mode (showing a single character per test run)
    • diffcov = with diff-coverage report (showing difference in coverage compared to previous commit). Tests will run in brief mode
    • profile = no coverage testing, but code profiling instead. This must be invoked manually using the -e parameter

    Note: Due to possible ‘complications’ when setting up PyPy on systems without pyenv, pypy3 tests also will not be automatically run; you must invoke them manually.

  • qa

    Perform flake8 code style checking

  • docs

    Builds HTML documentation using Sphinx

Environment Variables


Used on non-native-Linux operating systems to specify tests to skip. Valid values:

  • mswin – when running tox on Microsoft Windows
  • wsl – when running tox on Windows Subsystem for Linux (WSL)

Different Python Versions

The tox configuration files have been created to cater for more than one Python versions safely: If an interpreter is not found for a certain Python version, tox will skip that whole testenv.

However, with a little bit of effort, you can have multiple Python interpreter versions on your system by using pyenv. General steps:

  1. Install pyenv from
  2. Install tox-pyenv from
  3. Using pyenv, install the Python versions you want to test on
  4. Create a .python-version file in the root of the repo, listing the Python interpreter versions you want to make available to tox (see pyenv’s documentation about this file)
  5. Invoke tox with the option --tox-pyenv-no-fallback (see tox-pyenv’s documentation about this option)

Indices and tables